Investigations have concluded that methane does not appear to be photochemically reactive in the atmospheric system and does not participate in smog formation. Since methane is “nonreactive,” and may in the future be excluded from the total unburned hydrocarbon emissions, an instrument was designed and developed (termed the “methane analytical system”) enabling methane emissions to be quantified separately from total unburned hydrocarbon emissions. The instrument employed gas chromatographic principles whereby a molecular sieve column operating isothermally separated methane from the nonmethane hydrocarbons. Direct on-line sampling occurred via constant volume sample loops. The effluent was monitored with a flame ionization detector.The instrument was fully calibrated (i.e., extremely linear response over a large concentration range) for use with Diesel engines as part of an ongoing alternative fuels research program. Methane emissions from a light-duty, multicylinder, indirect-injected Diesel engine fumigated with natural gas were measured on line using the methane analytical system. Methane emissions were found to range from as low as 250 ppm to a high of nearly 2%. The nonmethane hydrocarbon emissions were determined by subtracting the methane level from the total unburned hydrocarbon level. In the event that the federal engine certification procedures are changed to be based on nonmethane hydrocarbon emissions, a methane analytical system such as the one described here would have great utility.