[en] Partition coefficients of uranium between phenocrysts and their host groundmass have been determined by fission-track mapping. The minerals analyzed include plagioclase, K-feldspar, biotite, olivine, clinopyroxene and orthopyroxene. The data for all these minerals show that U is strongly partitioned into the liquid and only a small fraction of the total whole-rock U content is present in the major rock-forming minerals. In volcanic rocks, the bulk of U is usually contained in glass although in acid volcanic rocks a significant portion may also be present in the U-rich accessory minerals