Experiments in a subcritical assembly with pebble-bed high-temperature gas-cooled reactor fuel and hydrogen ranging between 0 and 14.4 vol% were carried out to study the effects of water ingress on reactivity and to test the accuracy of diffusion-code calculations for small subcritical systems. Special emphasis is given to an adequate description of the influence of the water ingress on the diffusion properties of the pebble-bed system. The agreement between experimental and theoretical results is in all cases better than ±0.01 in keff.