The spin–spin relaxation rate R 2 (=1/T 2 ) in hydrogel foams measured by use of a multiple spin echo sequence is found to be dependent on the echo time spacing. This property, referred to as R 2 -dispersion, originates to a large extent from molecular self-diffusion of water within internal field gradients that result from magnetic susceptibility differences between the gel and air phase. Another contribution to the R 2 relaxation rate is surface relaxation. Numerical simulations are performed to investigate the relation between the foam microstructure (the mean air bubble radius and standard deviation of the air bubble radius) and foam composition properties (such as magnetic susceptibilities, diffusion coefficient and surface relaxivity) at one hand and the R 2 -dispersion at the other hand. The simulated R 2 -dispersions of gel foam are in agreement with the measured R 2 -dispersions. By correlating the R 2 -dispersion parameters and simulated microstructure properties a semi-empirical relationship is obtained that enables the mean air bubble size to be derived from measured R 2 -dispersion curves. The R 2 -derived mean air bubble size of a hydrogel foam is in agreement with the bubble size measured with X-ray micro-CT. This illustrates the feasibility of using 1 H R 2 -dispersion measurements to determine the size of air bubbles in hydrogel foams and of alveoli in lung tissue.