This paper presents results from a study of the long-term behaviour of carbon/epoxy composites. The interactions between ageing in water and constant mechanical loads are described, first experimentally then using a simple modelling approach. An identification procedure for the model is carried out and test/model comparisons are discussed. The results show that a four-parameter Burgers model can provide a good fit of the experimental data. The analysis of the results indicates the impact of water diffusion on the viscoelastic behaviour with larger strains for both creep and recovery phases. Those changes tend to appear at the early stage of the moisture diffusion process and stabilize quite quickly. This article is part of the theme issue 'Ageing and durability of composite materials'.