We investigated the vertical profiles of 4-hydroxybenzoic (I), 4-hydroxy-3-methoxy-benzoic (II),trans-4-hydroxy-3-methoxycinnamic (III),trans-4-hydroxycinnamic acid (IV); and thecisacid of IV in a 2MNaOH-extractable fraction from a core sample of Horonobe peat soils, Japan. Amounts of compounds I, II, III, IV, and thecisacid of IV in the seven layers averaged 590, 1400, 1300, 5400, and 510 μg C/g C soil, respectively. Compound IV was dominant in amount and averaged 62% of all four phenolic acids quantified in all layers. Thecis/transratio of IV increased with the depth of the samples, with two exceptions. By dominant peat-forming plants, the samples were classified as three types of peats—low moor, transitional moor, and high moor. We found that a relationship between phenolic acids and their humification should be discussed separately in their peat environment. The amount of compound IV in a high moor environment increased with an increasing degree of humification. We also determined phenolic acids in the ethyl-acetate-extractable fraction. The ethyl-acetate-extractable forms accounted for less than 11% of 2MNaOH-extractable phenolic acids (I + II + III + IV) quantified in all layers.