This paper presents the ultrasonic measurements of the elastic constants of RbCl, RbBr, and RbI as a function of pressure up to approximately 4 kbars in the temperature range of 120° to 300°K. The reduction of the raw data is analyzed according to the self‐consistent method suggested by Cook and is discussed at length so as to clarify the limitations as well as the precision of the final results presented in this paper. The results of adiabatic as well as the isothermal values of the elastic‐constant measurements at pressure are given together. Pressure derivatives of the elastic constants of the above salts under different thermodynamic conditions are calculated and are given separately. Finally the Cauchy relation and anisotropy are discussed.