We report on the gasification of guaiacol and phenol in supercritical water at 400-700 degrees C. The reactions were conducted in sealed quartz tubes, at times with added Ni wires that ran the entire length of the reactor. These reactors allowed the homogeneous and heterogeneous SCWG rates to be quantified separately for the first time. Guaiacol is mainly gasified into hydrogen, carbon dioxide, carbon monoxide, and methane. The rest of the guaiacol decomposes to phenol and o-cresol or reacts to form char. Nickel does not affect the conversion of guaiacol to phenol and o-cresol, but it significantly changes the gas product compositions. Phenol is mainly gasified into hydrogen, carbon dioxide, and methane. Hydrogen was the most abundant product at high phenol conversions. The gas compositions measured experimentally were largely consistent with those anticipated from chemical equilibrium calculations. In the absence of nickel, phenol conversions up to 68% were reached after 1 h. In the presence of Ni wire, complete conversion was obtained within 10 min. These results show that homogeneous, uncatalyzed gasification in supercritical water is slow, but rates are greatly increased by added Ni. The pseudo-first-order rate constant at 600 degrees C for homogeneous gasification of phenol is 3.0 (+/- 0.4) x 10(-4) s(-1), and the rate constant for Ni-catalyzed gasification is 1.1 (+/- 0.1) x 10(-3) cm/s.