| Marx to Engels. 7 January |
| Engels to Marx. 8 January |
| Marx to Engels. 22 January |
| Engels to Marx. 25 January |
| Marx to Engels. 27 January |
| Engels to Marx. 29 January |
| Marx to Hermann Becker. c. 1 February |
| Marx to Engels. 3 February |
| Engels to Marx. 5 February |
| Marx to Hermann Becker. 8 February |
| Marx to Engels. 10 February |
| Marx to Engels. 11 February |
| Engels to Marx. 12 February |
| Engels to Marx. 13 February |
| Marx to Engels. 23 February |
| Marx to Engels. 24 February |
| Engels to Marx. 25 February |
| Engels to Marx. 26 February |
| Marx to Engels. 26 February |
| Engels to Marx. 26 February |
| Engels to Marx. 27 February |
| Engels to Marx. 28 February |
| Marx to Hermann Becker. 28 February |
| Marx to Engels. 1 March |
| Marx to Engels. 8 March |
| Engels to Marx. 10 March |
| Engels to Marx. 17 March |
| Marx to Engels. 17 March |
| Engels to Marx. 19 March |
| Marx to Engels. 22 March |
| Marx to Engels. 31 March |
| Marx to Engels. 2 April |
| Engels to Marx. 3 April |
| Marx to Hermann Becker. 9 April |
| Engels to Marx. 11 April |
| Marx to Engels. 15 April |
| Engels to Marx. 15 April |
| Engels to Marx. 1 May |
| Engels to Wilhelm Wolff. 1 May |
| Marx to Engels. 3 May |
| Marx to Engels. 5 May |
| Engels to Marx. c. 6 May |
| Engels to Marx. 8 May |
| Engels to Marx. 9 May |
| Engels to Marx. 15 May |
| Marx to Engels. 16 May |
| Engels to Marx. 19 May |
| Marx to Engels. 21 May |
| Engels to Marx. 23 May |
| Engels to Marx. 27 May |
| Marx to Engels. 28 May |
| Marx to Roland Daniels. 2nd half May |
| Marx to Engels. 16 June |
| Engels to Joseph Weydemeyer. 19 June |
| Engels to Marx. 27 June |
| Marx to Joseph Weydemeyer. 27 June |
| Engels to Marx. c. 6 July |
| Engels to Ernst Dronke. 9 July |
| Marx to Engels. 13 July |
| Marx to Engels. c. 17 July |
| Engels to Marx. 17 July |
| Engels to Marx. c. 20 July |
| Engels to Marx. 30 July |
| Marx to Engels. 31 July |
| Engels to Marx. c. 1 August |
| Marx to Joseph Weydemeyer. 2 August |
| Engels to Wilhelm Wolff. 6 August |
| Engels to Joseph Weydemeyer. 7 August |
| Marx to Engels. 8 August |
| Engels to Marx. c. 10 August |
| Engels to Marx. c. 11 August |
| Marx to Engels. 14 August |
| Marx to Hermann Ebner. 15-22 August |
| Marx to Engels. c. 20 August |
| Engels to Marx. 21 August |
| Marx to Engels. 25 August |
| Engels to Marx. c. 27 August |
| Marx to Engels. 31 August |
| Engels to Marx. 1 September |
| Engels to Marx. 8 September |
| Engels to Marx. 11 September |
| Marx to Joseph Weydemeyer. 11 September |
| Marx to Engels. 13 September |
| Engels to Marx. 19 September |
| Engels to Marx. 23 September |
| Marx to Engels. 23 September |
| Engels to Marx. 25 September |
| Engels to Marx. 26 September |
| Marx to Amalie Daniels. 4 - 8 October |
| Marx to Engels. 13 October |
| Engels to Marx. 15 October |
| Marx to Joseph Weydemeyer. 16 October |
| Marx to Engels. 19 October |
| Marx to Engels. 25 October |
| Engels to Marx. c. 27 October |
| Marx to Joseph Weydemeyer. 31 October |
| Marx to Engels. 24 November |
| Engels to Marx. 27 November |
| Marx to Adolf Cluss. Early December |
| Marx to Engels. 1 December |
| Marx to Hermann Ebner. 2 December |
| Engels to Marx. 3 December |
| Marx to Engels. 9 December |
| Engels to Marx. 10 December |
| Engels to Marx. 11 December |
| Engels to Marie Blank. 15 December |
| Engels to Marx. 16 December |
| Engels to Jenny Marx. 18 December |
| Marx to Joseph Weydemeyer. 19 December |
| Marx to Ferdinand Freiligrath. 27 December |