A criterion of equilibrium development for the sublimation processes has been for-mutated and theoretically substantiated. It is based on the ratio of sublimation temperature, Tub (which corresponds to the vapor partial pressure of 10(-7) bar), to the enthalpy of sublimation, DeltaH(T)(o). This ratio is in the range 3.6 +/- 0.2 K mol kJ(-1) (S.D.). This criterion has been applied to the free-surface decompositions of 100 different substances including some metalloids and simple binary compounds together with metal salts of inorganic and organic acids, ammonium and hydrated salts. The experimental values of the initial temperature, T-in, and the E parameter, which are equivalent to the theoretical T-sub and DeltaH(T)(o) values, were taken from the literature. The mean value of T-in/E ratio for all reactants is equal to 3.6+/-0.4 (S.D.). This means that the decomposition of all solids proceeds in agreement with the equilibrium laws. A higher deviation of T-in/E ratio from the mean (compared to theory) is connected with random errors in the determination of the E parameter and the uncertainty in the definition of the initial temperature of decomposition. For the first time, the third-law method was used for the purposeful calculation of the E parameters in cases of free-surface solid decompositions. The comparison of results obtained by the second- and third-law methods for 20 different reactants revealed the great advantages of the latter method in precision and accuracy of determinations and also in time spent for the experiment.