1. Human liver sulphotransferase and UDP-glucuronosyltransferase were studied with phenol, methyl-, ethyl-, propyl-, butyl-, phenyl-, nitro-, amino-phenols and hydroxybenzoic acids as substrates. 2. The Michaelis-Menten constants (Km) and the maximum velocities of reaction (Vmax) of sulphotransferase and UDP-glucuronosyltransferase for each substrate were measured. 3. The Km values for sulphotransferase varied over 5000-fold whereas they varied over 25-fold for UDP-glucuronosyltransferase. 4. Sulphotransferase and UDP-glucuronosyltransferase have different structure-activity relationships with phenolic substrates.