The ionized (S-) and non-ionized (SH) phenyl (PSH) and methyl (MSH) salicylates exhibit strong and undetectable absorption at 350 nm, respectively. These molecular absorption properties were used to determine the binding constants of S-and SH with the micelles of sodium dodecyl sulphate in terms of the pseudophase model of micelles. The binding constants (K ) of Sare 4.0 f 2.0 dm' mol -' for PSH and 5.7 f 1-2 dm3 mol-' for MSH and those (K2) of SH are 2350 f 90 dm mol-' for PSH and 100 f 10 dm3 mol-' for MSH at 30 "C. 5