The dissociation constants of monochlorophenols (2-, 3-, 4-chlorophenols) were examined using direct UV-vis spectroscopy at temperatures from 25 to 175degreesC and at saturated vapor pressures in a high-temperature, high-pressure cell. The dissociation constant, K-a increased under experimental temperatures in the order: 2- chlorophenol, 3-chlorophenol, and 4-chlorophenol. The dissociation constant of 4-chlorophenol increased with increasing temperature under experimental conditions, while those of 2- and 3-chlorophenol reached maximum values at around 125degreesC, and then decreased with further increases in temperature. The slope of partial derivative(log K)/partial derivative (1/T) was nonconstant and positive, that is, endothermic, below 150degreesC. The data on dissociation constants were analyzed by simultaneous regression to yield a five-term equation that described the Van't Hoff isobar. The magnitude of enthalpy DeltaH increased at 25degreesC in the order: 3- chlorophenol, 4-chlorophenol, and 2- chlorophenol. The decrease in enthalpy at the absolute temperature was larger for 3- chlorophenol than for either 2- or 4-chlorophenol. Considering the equilibrium constant K-b for the isocoulombic reaction of monochlorophenol with OH-, the nearly linear relationship of log K-b vs. 1/T for temperatures between 25 and 175degreesC indicates that the DeltaC(p) values for this isocoulombic reaction are low.