| Marx to Ludwig Kugelmann. 19 January |
| Engels to Wilhelm Liebknecht. 27 January |
| Engels to Friedrich Adolph Sorge. 14 February |
| Engels to Wilhelm Blos. 21 February |
| Engels to Friedrich Adolph Sorge. 27 February |
| Marx to George Moore. 26 March |
| Marx to George Moore. 28 March |
| Marx to Jenny Marx. 19 April |
| Marx to Jenny Longuet. 20/24 April |
| Marx to Maurice Lachatre. 12 May |
| Marx to Ludwig Kugelmann. 18 May |
| Engels to Gottfried Ermen. 1 June |
| Marx to Ludwig Kugelmann. 24 June |
| Marx to Engels. 15 July |
| Engels to Marx. 21 July |
| Marx to Maurice Lachatre. 23 July |
| Engels to Jenny Longuet. 2 August |
| Marx to Engels. 4 August |
| Marx to Friedrich Adolph Sorge. 4 August |
| Marx to Ludwig Kugelmann. 4 August |
| Marx to Ludwig Kugelmann. 10 August |
| Engels to Marx. 12 August |
| Marx to Engels. 14 August |
| Marx to Jenny Longuet. 14 August |
| Marx to Engels. 1 September |
| Engels to Marx. 5 September |
| Engels to Friedrich Adolph Sorge. 12 September |
| Marx to Engels. 18 September |
| Marx to Max Oppenheim. 20 September |
| Engels to Marx. 21 September |
| Engels to Laura Lafargue. 15 October |
| Marx to Max Oppenheim. 17 October |
| Engels to Hermann Lopatin. c. 20 October |