pKa values have been determined for ten acidic herbicides and one fungicide using a spectrophotometric method, pKa values are reported for [(4-chloro-o-tolyl)oxy]acetic acid (MCPA), 3.13; (±)2-[(4-chloro-o-tolyl)oxy]propionic acid (mecoprop), 3.11; (2,4-dichlorophenoxy)acetic acid (2,4-D), 2.87; 2-(2,4-dichlorophenoxy)propionic acid (dichlorprop), 2.86; (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T), 2.85; 2-(2,4,5-trichlorophenoxy)propionic acid (fenoprop), 2.83; 3,6-dichloro-o-anisic acid (dicamba), 1.90; 2-sec-butyl-4,6-dinitrophenol (dinoseb), 4.62; 4,6-dinitro-o-cresol (DNOC), 4.46; 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil), 4.20; and pentachlorophenol (PCP), 4.71. The pKa values for (o-chlorophenoxy)acetic acid (2-CPA), 3.00, and (p-chlorophenoxy)acetic acid (4-CPA), 3.05, have been included for comparative purposes. The pKa values for 2-CPA, 4-CPA, MCPA, 2,4-D, and 2,4,5-T from this study were in close agreement with those previously reported from potentiometric and conductimetric determinations. The pKa values for 4-[(4-chloro-o-tolyl)oxy]butyric acid (MCPB), 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB), 4-amino-3,5,6-trichloropicolinic acid (picloram), 3,6-dichloropicolinic acid (M-3723), and 2,3,5-triiodobenzoic acid (TIBA) could not be determined using the spectrophotometric method. © 1978, American Chemical Society. All rights reserved.