The densities and compositions of both phases in the methane-n-heptane solubility equilibrium have been determined at 25°, 55°, and 85 °C. at total pressures from 35 to 250 atm. The critical pressures of complete miscibility were found, and the properties of the system are discussed. The nitrogen-n-heptane system was investigated similarly at 100.9 atm. and temperatures from 25° to 115 °C. The system composed of Viking natural gas and impure heptanes was also investigated at 25 °C. The three systems, methane-pentane, -hexane, and -heptane are compared and discussed.