Variations in the visible photoacoustic spectra of carbon black samples from different sources have been observed. These variations indicate that some care must be exercised in the choice of a particular carbon black for use as a reference (perfectly absorbing) material in photoacoustic spectroscopy, particularly when quantitative information from weakly absorbing samples is desired. Of the carbon black samples investigated, Norit A decolorizing carbon most closely approximates a perfect absorbing material.