The dissociation equilibrium of 4-methylphenol in aqueous solutions has been studied spectrophotometrically over the temperature range 298 to 448 K; at higher temperatures the anion decomposed appreciably. The solutions that were studied had different ratios of weak acid to conjugated base, and the ionic strength was varied with sodium trifluoromethane sulfonate between 0.0 mol (.) kg(-l) and 0.3 mol (.) kg(-1). The overall analysis of the optical absorption of the solutions in the range 250 to 380 nm was based on the corresponding isocoulombic reaction. We report the values of the thermodynamic quantities at 298.2 K for the two equilibria. The results also suggest that 4-methylphenol may be used as an indicator to study equilibria of other weak aqueous electrolytes.