In this investigation, there was deduced from experiments that an exothermic reaction in the thermal runaway process in alkaline batteries is the electrochemical reaction of atomic hydrogen recombination H-ads (Cd) + H-ads (Ni) -> H-2 up arrow (H2O + H-ads + e(-) -> H-2 up arrow + OH- on a cathode and H-ads + OH- -> H2O + e(-) on an anode). A rate-limiting step for this reaction on both a cathode and an anode is found to be a step of metal-hydrides disintegration. This electrochemical reaction proceeds at voltage 0.5-0.6 V on battery terminals according to experimental data on the thermal runaway. The possible thermal runaway mechanism has been proposed corresponding to all the experimental results obtained. (C) The Author(s) 2015. Published by ECS. All rights reserved.