In order to evaluate t he effects of hydrochloric acid and neutral salts on the absorption of light by an organic acid dye, spectrophotometric measurements were made of a series of solutions of hydrochloric acid and of neutral salts plus hydrochloric acid, each containing 3 X 10-4 M I'l-naphthoquinonesulfonic acid.The dye in these solutions showed small differences in spectral absorption. Similar differences may contribute to the changes of absorption shown by true indicators . Most of the data were compatible with the assumption that new absorption bands were not produced or that there was no considerable shift in the position of the original bands with respect to wave number.The increase in the absorption by the dye in the presence of salts was proportional to the square root of the m olar activity of the electrolyte. The proportionality factor was different for each salt and varied from 0.144 to 0.353.The results suggest that the specific effects of ions on the absorption indices of positive, neutral, and negative forms of an indicator must be taken into account, as well as the normal effects of the ions on the dissociation of the indicator, for accurate pI-I measurements.