A survey was undertaken in S.W. England to study epiphyte differentiation between free-standing oak (Quercuspetraea) and ash (Fraxinus excelsior) trunks. Epiphyte cover was measured on 23 trees in 87 15 × 15 cm quadrats positioned on the north, east, south and west faces of each trunk at a height of 1.52 m. Measurements of 20 environmental factors were made at each quadrat position. Analysis of the vegetation data by reciprocal averaging showed that floristic variation is continuous and corresponds only partially to phorophyte identity. Rank correlation coefficients were calculated between the floristic axis obtained by reciprocal averaging, and individual environmental factors including a variate, obtained by principal components analysis of the environmental data, which expressed the major differences between the oak and ash environments. The most significant correlation was obtained with bark pH. Values of bark pH for oak and ash samples overlapped considerably although the mean for ash (5.1) is higher than for oak (4.7). It is concluded that the epiphytic flora is responding to variations in bark acidity or some closely related factor and not to other, closely phorophyte dependent nutritional factors. Factors influencing bark acidity in oak and ash are briefly discussed.