At pH = 5.0, the degradation of pentachlorophenol (PCP) by the photo-Fenton reaction was facilitated in the presence of humic acid (HA). To identify the functional groups in HA that contributed to this facilitation, the degradation kinetics of PCP were investigated for six types of phenolic acids. The pseudo-first-order rate constants of PCP degradation (k3) increased with those for the photoreduction of Fe(III) (k1). This indicates that the photoreduction of Fe(III) is an important step in the degradation of PCP by the photo-Fenton reaction. Moreover, the k3 values for gallic, protocatechuic and caffeic acids were larger than those for syringic, vanillic and ferulic acids. This shows that the phenolic acids in HA, which contain neighboring hydroxyl groups, are major contributors to the facilitation of PCP degradation.