The problem of numerically determining the parameters which describe an analytical transmission function for gaseous molecular absorption over an infrared band is addressed. A double exponential described by three absorber parameters and one spectral parameter is proposed as the transmission function. Four numerical methods for the determination of the parameters are briefly discussed, and applied to line-by-line and measured spectra for nitrous oxide (N20) over its three principal infrared bands. A discussion is also presented on a comparison between the line-by-line and the laboratory measured spectra. The model developed with the recommended method reproduced the original data with an overall rms deviation of 1.49%, which is an improvement over a previously reported model. The model parameters at 5 cm -1 intervals for 20 cm-1 resolution transmitt ance calculations throughout the bands, are available upon request in a form compatible with the LOWTRAN code.