This paper described a theoretical investigation into the variations of the specific heats of airCpandCv, and their ratio γ, with relative humidity. The investigation is based on published equations forCp, and the predicted values are derived from theoretical and experimental thermodynamic data on the constituents of humid air. For acoustical applications, such as the determination of the variation of the speed of sound in humid air, the perturbation of the term γ/M̄ with relative humidity is also presented.