Wet air oxidation (WAO) reactions of cellobiose, phenol, and syringic acid were carried out under mild conditions (155 degrees C; 0.93 MPa O-2; soluble catalyst, Na-5[PV2Mo10O40]) Initial oxidation rates were rapid but decreased to small values as less reactive oxidation products accumulated. Recalcitrant oxidation products were consumed more rapidly, however, if additional cellobiose or phenol was added, a procedure equivalent to recirculation of WAO-reactor effluent. A half-fraction, factorial investigation was used to correlate rate enhancement with individual experimental variables: using a two-level design, effluents from the WAO of cellobiose were combined with unreacted cellobiose and WAO resumed. The effluents were degraded more rapidly than could be accounted for by residence time alone. Correlations identified by the factorial investigation were consistent with oxidation of recalcitrant compounds by highly reactive intermediates generated during the initially rapid radical-chain autoxidation of unreacted cellobiose.