jats:p Carbon-fibre-reinforced polymer (CFRP) tendons can be used as a corrosion-resistant alternative to steel for reinforcing or prestressing concrete in aggressive marine environments. The design lives of many civil marine structures often span decades and so the long-term durability of the internal reinforcement is important. One significant, yet overlooked, aspect of CFRP tendon durability is its susceptibility to swell on absorption of aqueous solutions including water and salt water. This paper reviews uptake-induced swelling in CFRP materials and draws comparisons with studies of swelling caused by thermal effects. The diffusion of aqueous solutions and their interaction with the polymer matrix are identified as the primary mechanisms responsible for the swelling. Experimental details are presented that correlate the uptake of water and salt water to swelling observed in CFRP tendons. The results are contrasted with both uptake- and thermal-driven behaviour cited in the literature. Lamé's equations are used to estimate the magnitude of the resulting stresses induced in the surrounding concrete. The practical significance of the findings is discussed, including the potential for cracking in the surrounding concrete cover and the implications for the tendon–concrete bond. </jats:p>