A computer program package AQUARY has been developed for quantitative evaluation of concentration distributions of the radiolytic species in overall boiling water reactor primary systems. The hydrogen peroxide decomposition rate k, the gas release coefficient ϵ, and the accumulation of products through recirculation of the coolant were taken into consideration. The following relations were found: 1. Hydrogen and hydrogen peroxide concentrations in the core are substantially high, and the following relation holds in the core, 2[O2] = [H2] < [H2O2]. 2. The hydrogen peroxide concentration contributes markedly to the oxygen concentration at the water sampling stations in a plant. In particular, the following equation holds if k > 0.1 s-1, 2[O2] at the sampling station = [H2O2] at the core exit.