The quantity, accumulation rate and radiation–chemical yield of the molecular hydrogen received at radiation and heterogeneous transformation of water under an influence of gamma-quanta (⁶⁰Co, P=18.17 rad/s, T=300 K) on the system of BeO (with a particle size of d<4, 32–53 and 75–106 µм)+adsorbed water have been investigated. According to the particles sizes of d<4, 32–53 and 75–106 µм the maximum radiation – chemical yield (at values of level of filling of a particles surface θ=4) of molecular hydrogen makes G(H₂)=6.4, 3.81 and 2.8 molecules/100eV. The made experiments show that in the system of micro BeO (with a size of particles of d2O, the radiation –chemical yield of molecular hydrogen is directly proportional to level of filling of a particles surface at θ<2, and at θ ≥2 the growth slows down.