A method and apparatus is provided for the analysis of gaseous compounds, especially for determining the concentration of a gas or gases in a gas mixture by microwave spectroscopy. Microwave radiation is generated at one or more frequencies the gas is most responsive to, transmitted by antenna, passed through the gas under test, received by antenna, and the absorption and/or reflection of the microwave radiation is measured by means such as digitization and analysis using the FFT spectrum versus energy response generated, the response subsequently used to calculate the gas concentration.