The absorption coefficients for methane and nitrous oxide have been measured at 17 DF laser wavelengths in the region of 3.8 &mgr;m using a spectrophone. Concentrations of the absorbing gases very near their atmospheric concentrations were used. The absorption coefficients for the two gases at standard sea level atmospheric conditions are presented. The spectrophone device developed for this work is a differential cell which produces an equivalent absorption background signal of only 3.3×10−7m−1/W.