Densities of dilute aqueous solutions of CH4, CO2, H2S, and NH3 were determined experimentally at temperatures ranging from 298.15 K to 705 K and pressures to 35 MPa by means of a vibrating tube densitometer. Appropriate techniques were developed for preparation, storing, and handling of solutions as well as their concentration determination. The apparent molar volumes were calculated and compared with available literature data. The temperature, pressure, and concentration variation of apparent molar volumes in the sub- and supercritical regions is discussed qualitatively.