Dissolution followed by hydrolysis of geraniin during decoction of chopped dried herb of G. thunbergii [a medicinal plant] was determined by high-performance liquid chromatography on a column of hydrocarbon polymer. The effects of using vessels of different materials for decoction, on dissolution and hydrolysis of geraniin, were determined. Upon slow warming of water, the amount of geraniin in the decoction reaches the maximum in a short time after the water starts to boil, and the amount of geraniin then decreases fairly fast. The amount of corilagin, a product of geraniin hydrolysis, increases at the same time and continues to increase for some time after most geraniin disappears. When decoction was performed in beakers of approximately the same volume and of different materials, no marked difference in the results was noted. Appreciable differences were observed between an aluminum kettle and an earthen vessel of approximately the same volume and of considerably different weight. The traditional way of decoction of crude drugs indicates that use of metallic vessels should be avoided; the superiority of the earthenware is due to mild warming. Experiments with infusing chopped dried herb of G. thunbergii repeatedly in a teapot with hot water showed that this is a good method of extracting geraniin without causing extensive hydrolysis.