Effect of pH variation on the relative astringency (RA) was investigated to improve reproducibility of RA values and to measure variations of tannin activity by difference in tannin structure. The calibration curves of J.P. tannic acid obtained at various pH showed the best sensitivity, linearity, and reproducibility, at pH 6.0. The calibration curves of geraniin also showed best properties and the best analogy to J.P. tannic acid, at pH 6.0. Marked difference of calibration curves between J.P. tannic acid and geraniin was observed when pH of the test solution was significantly different from 6.0. The RA values measured at various pH by the calibration curve of J.P. tannic acid at pH 6.0 showed that the hydrolysis of geraniin, and also of Geranium thunbergii extracts in boiling water induce slow decrease of the activity at pH 6.0, and marked temporary increase of RA at lower pH. This property could be characteristic of geraniin since RA of J.P. tannic acid measured at various pH showed only slow decrease after hydrolysis. The RA values obtained at pH 6.0 (RA_pH6) were measured for seasonal variation of tannin content in G. thunbergii, for tannin contents of Geranium species and of other tannin-containing plants, to give results somewhat different from those obtained without pH regulation. The RA values based on the calibration curve of geraniin (RAG), which should give better reproducibility of the data because of homogeneity of geraniin, were also obtained for the above samples.