A spectrophotometric study is presented on the first ionization equilibrium of a class of substituted sulfonephthaleins, whose second ionization was the subject of the first part of this work. The present study was more difficult than the previous in that highly acid media and acidity functions had to be used. Nevertheless the results were of sufficient accuracy to allow the dual substituent analysis of Swain and Lupton (C.G. Swain and A.C. Lupton, Jr., J. Am. Chem. Soc., 90 (1968) 4328). Generally speaking, the dependences of equilibrium and spectral parameters on field and resonance parameters found in this and the previous paper were very similar.