We have classified over 1500 infrared spectra obtained with the PHT-S spectrometer aboard the Infrared Space Observatory according to the system developed for the Short Wavelength Spectrometer (SWS) spectra by Kraemer et al. The majority of these spectra contribute to subclasses that are either underrepresented in the SWS spectral database or contain sources that are too faint, such as M dwarfs, to have been observed by either the SWS or the Infrared Astronomical Satellite Low Resolution Spectrometer. There is strong overall agreement about the chemistry of objects observed with both instruments. Discrepancies can usually be traced to the different wavelength ranges and sensitivities of the instruments. Finally, a large subset of the observations (250 spectra) exhibit a featureless, red continuum that is consistent with emission from zodiacal dust and suggest directions for further analysis of this serendipitous measurement of the zodiacal background.