An approach is proposed for analysing the deviations of the heat capacity Cp(T) of solid solutions from the Kopp–Neumann rule (KNR) ΔC(T) = Cp(T) − CKNR(T). Temperature dependences of the heat capacity Cp(T) of selected compositions of systems (InP)x (InAs)1−x and (GaAs)x (InAs)1−x at temperatures of 5–300 K are analysed in the Debye–Einstein approximation. It was established that in the case of substitution of atoms in the cation subsystem (Ga3+ ↔ In3+) with the same subsystem of anions (As3−), the positive values of ΔC(T) at T 100 K are the result of a decrease in the fraction of the Debye contribution without changing the upper limit of the oscillation frequency. In the case of substitution in the cation subsystem (P3− ↔ As3−) with the invariant cation subsystem (In3+) to the low-temperature positive contribution of the additional low-frequency Einstein mode, a positive part is added from the modified Debye mode having the characteristic temperature θD below the additive value θDKNR. The adequacy of this model is confirmed by Raman scattering data.