Abstract: A method and device have been developed which accurately measure the BTU content of hydrocarbons at pipeline conditions. The cost of this device allows some users who currently collect samples to have the advantages of on-line BTU measurement. The device measures, reports, and logs the BTU value per unit volume several times per minute. As the absorption cell is nearly isobaric with the pipeline, the energy flow through the pipeline can be computed directly as the product of the BTU value per unit volume times volumetric flow rate. BTU and composition are measured via optical absorption spectroscopy in the vicinity of 900 nm using a spectrograph and a silicon array detector. In contrast to longer infrared wavelengths, the natural gas absorption spectrum in this spectral region is a linear combination of the spectra of the component species. The observed spectrum can therefore be accurately fit to yield the concentration, in molecules per unit volume, of all C₁ to C₆ hydrocarbon components and water. The energy content of the components is then summed to yield the BTU per unit volume. Additional information such as the compressibility, density, and relative density can also be obtained from the measured gas compositions.