The impact that product gas in the reaction atmosphere has on the kinetics of the thermal decomposition of inorganic solids is an outstanding issue that requires a solution to understand the reactions in a solid–gas system. Among a variety of kinetic phenomena induced by atmospheric gas, the restraining effect of the overall reaction rate with increasing partial pressure of the product gas in the reaction atmosphere is the most widely observed phenomenon. In this study, we describe the universal kinetics of the thermal decomposition of solids over different temperatures and partial pressures of the gas, as exemplified by the thermal decomposition of Cu(OH)₂. Universal kinetic descriptions were enabled by introducing an accommodation function, with respect to atmospheric water vapor pressure, into the fundamental kinetic equation for solid-state reactions. The thermoanalytical curves as measured systematically under different temperatures and water vapor pressure conditions were kinetically analyzed in a step-by-step manner to attain kinetic modeling of the physico-geometrical consecutive process that comprises the induction period, surface reaction, and phase boundary-controlled reaction. The impact that atmospheric water vapor has on the kinetics of each physico-geometrical reaction step was separately evaluated using the universal kinetic approach.