The effect of organic sulfonic acids as catalysts during phenol liquefaction of Pinus radiata bark was investigated using five organic sulfonic acids [benzenesulfonic acid (BSA), methanesulfonic acid (MSA), 1,5-naphthalenedisulfonic acid (NDSA), 1-naphthalenesulfonic acid (NSA), and p-toluenesulfonic acid (PTSA)] and sulfuric acid (SA) as a reference. All five organic sulfonic acids were very effective catalysts for phenol liquefaction of the bark, with almost complete liquefaction being achieved. PTSA was even effective for liquefaction at a liquor ratio (phenol/bark) as low as 2. The average molecular weights of the liquefied materials obtained from phenol-SA and phenol-PTSA systems were very similar at the same liquefaction time. IR spectroscopy confirmed that the residue remaining after acid-catalyzed phenol liquefaction of the bark was very similar to cellulose. The crystallinity of the residue when using PTSA as catalyst was slightly lower that that when using SA. The combined phenol content was higher using SA than using PTSA at the same liquefaction yield. The results demonstrate that these organic sulfonic acids play an important role in retarding the condensation reaction between phenol and bark components during the acid-catalyzed phenol liquefaction.