Isotopic distributions are frequently used as part of the peak assignment process in the processing of mass spectra. The best methods for producing accurate peak assignments must account for the peak shape and resolving power. In other words, the full profile of the isotopic distribution is important. Conventional methods for modeling isotopic distributions generally assume a peak profile that is not applicable to fully apodized absorption mode spectra because the peak shapes in these spectra are distinctly different from those seen in normal (i.e., magnitude mode) spectra. We present results illustrating this problem and describe a method for producing more accurate isotopic distribution models for this class of spectra.