| Marx to Hermann Jung. 8 January |
| Engels to Marx. 9 January |
| Engels to Marx. 11 January |
| Engels To Marx. January 19, 1870 |
| Marx to Engels. 22 January |
| Marx to Cesar De Paepe. 24 January |
| Engels to Marx. 25 January |
| Marx to Engels. 26 January |
| Engels to Marx. 1 February |
| Engels to Karl Klein and Friedrich Moll. 8 February |
| Engels to Marx. 9 February |
| Marx to Engels. 10 February |
| Engels to Marx. 11 February |
| Marx to Engels. 12 February |
| Engels to Marx. 17 February |
| Marx to Ludwig Kugelmann. 17 February |
| Marx to Engels. 19 February |
| Marx to Charles Dobson Collet. 19 February |
| Marx to Engels. 21 February |
| Engels to Marx. 22 February |
| Marx to Peter Imandt. 23 February |
| Marx to Engels. 5 March |
| Marx to Laura and Paul Lafargue. 5 March |
| Engels to Marx. 7 March |
| Engels to Rudolf Engels. 8 March |
| Marx to Engels. 9 March |
| Marx to Engels. 10 March |
| Engels to Marx. 13 March |
| Engels to Jenny Marx (daughter). 17 March |
| Marx to Engels. 19 March |
| Engels to Marx. 21 March |
| Marx to Engels. 24 March |
| Marx to Wilhelm Bracke. 24 March |
| Marx to Philippe Coenen. 24 March |
| Marx to Engels. 26 March |
| Marx to Ludwig Kugelmann. 26 March |
| Engels to Marx. 27 March |
| Engels to Marx. 28 March |
| Marx to Ludwig Kugelmann. 28 March |
| Marx to Sigfrid Meyer and August Vogt. 9 April |
| Engels to Marx. 13 April |
| Marx to Engels. 14 April |
| Engels to Marx. 15 April |
| Marx to Paul Lafargue. 18 April |
| Engels to Marx. 19 April |
| Marx to Engels. 19 April |
| Marx to Paul Lafargue. 19 April |
| Engels to Marx. 21 April |
| Marx to Engels. 28 April |
| Engels to Willhelm Bracke. 8 April |
| Engels to Marx. 29 April |
| Marx to Engels. 29 April |
| Engels to Marx. 1 May |
| Marx to Engels. 7 May |
| Engels to Marx. 8 May |
| Marx to Ludwig Kugelmann. About 8 May |
| Marx to Engels. 10 May |
| Engels to Marx. 11 May |
| Marx to Engels. 11 May |
| Engels to Marx. 15 May |
| Marx to Engels. 16 May |
| Engels to Marx. 17 May |
| Marx to Engels. 18 May |
| Engels to Marx. 19 May |
| Marx to his Daughter Jenny. 31 May |
| Marx to Victor Le Lubez. 14 June |
| Marx to Kugelmann. 27 June |
| Marx to Engels. 5 July |
| Engels to Marx. 6 July |
| Marx to Hermann Lopatin. 6 July |
| Engels to Marx. 7 July |
| Marx to Engels. 8 July |
| Marx to Hermann Jung. 14 July |
| Marx to Engels. 20 July |
| Engels to Marx. 22 July |
| Marx to Eugen Oswald. 26 July |
| Marx to Engels. 28 July |
| Marx to Paul and Laura Lafargue. 28 July |
| Marx to Engels. 29 July |
| Marx to Wilhelm Liebknecht. 29 July |
| Engels to Marx. 31 July |
| Marx to Engels. 1 August |
| Marx to Johann Philipp Becker. 2 August |
| Marx to Eugen Oswald. 2 August |
| Engels to Marx. 3 August |
| Marx to Engels. 3 August |
| Marx to Eugen Oswald. 3 August |
| Marx to Engels. 4 August |
| Engels to Marx. 5 August |
| Marx to Eugen Oswald. 5 August |
| Marx to Hermann Jung. 6 August |
| Marx to Eugen Oswald. 7 August |
| Marx to Engels. 8 August |
| Engels to Marx. 10 August |
| Marx to Engels. 12 August |
| Marx to Hermann Jung. 12 August |
| Marx to Engels. 15 August |
| Engels to Marx. 15 August |
| Engels to Jenny Marx. 15 August |
| Marx to Engels. 17 August |
| Engels to Marx. 20 August |
| Marx to Engels. 22 August |
| Marx to Engels. 30 August |
| Marx to Friedrich Adolph Sorge. 1 September |
| Marx to Engels. 2 September |
| Marx to Sigfried Meyer. 2 September |
| Engels to Marx. 4 September |
| Marx to Eugen Oswald. 4 September |
| Marx to Engels. 6 September |
| Engels to Marx. 7 September |
| Marx to Engels. 10 September |
| Engels to Marx. 12 September |
| Marx to Edward Spencer Beesly. 12 September |
| Engels to Marx. 13 September |
| Marx to Engels. 14 September |
| Marx to Johann Philipp Becker. 14 September |
| Marx to Cesar De Paepe. 14 September |
| Marx to Ludwig Kugelmann. 14 September |
| Marx to Eugen Oswald. 14 September |
| Marx to Johann Philipp Becker. 15 September |
| Marx to Engels. 16 September |
| Marx to Edward Spencer Beesly. 16 September |
| Marx to Eugen Oswald. 23 September |
| Marx to Edward Spencer Beesly. 19 October |
| Marx to Peter Imandt. 11 November |
| Marx to the Dutch and Flemish Internationals. 5/9 December |
| Marx to Ludwig Kugelmann. 13 December |
| Engels to Friederick Greenwood. 17 December |
| Engels to Natalie Liebknecht. 19 December |