It is well known that the chemical interaction between gas and solid occurs principally through two steps : surface associated processes (viz. adsorption, dissociation) and bulk dependent processes (viz. diffusion, bulk sorption). Earlier works on metallic alloy-hydrogen systems have shown that the hydrogen sorption rate by the getter alloy is decided by the surface related processes at low pressures and that it is diffusion dependent at higher pressures. In the series of batch conversion experiments conducted on cracking isotopic waters using a metallic (Zr-V-Fe) getter alloy, it has been observed that, over a wide range of pressure (1-500 Pa) and temperature (100-400 degrees C), the conversion reaction rate remains governed by surface processes (characterized by P-1 dependence of the absorption rate) at the beginning of the process, i.e., when the water vapor pressure is high, and that it shifts itself to become diffusion dominated (characterized by the P-1/2 dependence) at later times, i.e., when the water vapor pressure becomes low. This observed behavior is attributed to the amount and nature of change in the hydrogen and oxygen concentrations in the alloy occurring during the conversion process. This work presents the experimental observations and discusses the occurrence and relevance of shift in the rate-limiting step during the conversion process.