We examined the decomposition of o-, m-, and p-ethylphenol and o-, m-, and p-hydroxyacetophenone in dilute aqueous solutions at 460 degrees C and 25.3 MPa, both in the presence and absence of added oxygen. In the absence of oxygen, the ethylphenols produced vinylphenols as the major product and the hydroxyacetophenones produced phenol, benzendiols, and hydroxybenzaldehydes. In the presence of oxygen, ethylphenols and hydroxyacetophenones reacted through two major parallel paths and one minor path. The major primary paths for ethylphenols were to vinylphenols and to ring-opening products and ultimately CO2. The minor path was to phenol. For hydroxyacetophenones, the major primary paths were to phenol and to ring-opening pro ducts and ultimately CO2. The minor path was to hydroxybenzaldehydes. The relative rates of these parallel paths were sensitive to the location of the substituent. Although reactions did occur in the absence of oxygen, the disappearance rates were much slower than those observed during oxidation. Power-law global rate expressions were developed for reactant disappearance during oxidation. These rate laws were used along with rate laws previously reported for other monosubstituted phenols to examine the relative oxidation rates for different phenols. All of the substituted phenols oxidized more quickly than phenol itself. The oxidation rates for the substituted phenols were functions of both the identity and location of the substituent. For a given substituent, the reactivity was always in the order ortho > para > meta for all of the substituted phenols examined.