Abstract: A method and apparatus of determining the energy content of a hydrocarbon gas mixture having a diluent gas. The dielectric constant of the hydrocarbon gas mixture and diluent gas is measured, the gas mixture is then directed through a separation column which extracts a mixture containing the diluent gas. The dielectric constant of the extracted mixture is then measured, and the energy content of the hydrocarbon gas mixture and dilute gas is calculated from the dielectric measurements. After use, the separator column and dielectric measuring cell are purged of the residual gases.