Context. Laboratory simulations on interstellar or cometary ice analogues are crucial to understand the formation of complex organic molecules that are detected in the interstellar medium (ISM). Results from this work give hints on physical and chemical processes occurring in space and can serve as a benchmark for dedicated space missions. Aims. The aim of this work is to consolidate the knowledge of ice evolution during the star formation process by investigating the influence of thermal reactions as a source of molecular complexity in the ISM. In this study, we are interested in the thermal reactivity between two interstellar molecules, formaldehyde (H 2 CO) and methylamine (CH 3 NH 2) in water ice analogues. Methods. We used Fourier transform infrared spectroscopy, mass spectrometry, and B3LYP calculations to investigate the thermal reaction between formaldehyde and methylamine (14 N and 15 N) at low temperature in water ice analogues. Results. We demonstrate that methylamine and formaldehyde quickly react in water ice analogues for astronomically relevant temperatures and form N-methylaminomethanol CH 3 NHCH 2 OH. The measured activation energy of this reaction, 1.1 ± 0.05 kJ mol −1 (1.8 ± 0.08 kJ mol −1 with methylamine 15 N), allows the reaction to proceed in interstellar ices, when the ices are gently warmed, as it occurs in young stellar objects, in photo-dissociation regions, or in comets. Therefore, CH 3 NHCH 2 OH is likely to be found in these objects. This hypothesis is confirmed by numerical simulations that clearly show that the formation of N-methylaminomethanol is likely at low temperature. Isotopic experiments as well as photochemical studies have also been performed to better characterise the ice evolution induced by heat and ultraviolet radiation during star formation.