Application of a proton microprobe to experimentally produced mineral-liquid pairs has enabled determination of Nb and Ta partition coefficients (D) between garnet, clinopyroxene, and a Nb- and Ta-enriched hydrous basaltic liquid at 2.5 GPa, 1100°C. For garnet DNb=0.02, DTa=0.06, and for clinopyroxene DNb=0.005 and DTa=0.013. Both minerals significantly fractionate Nb and Ta from each other and may have an important role in modifying the Nb/Ta ratio of evolving magmas from that of their parent magma or their source region. For eclogite melting up to 40% change could occur in the Nb/Ta ratio. Additional partition coefficients obtained using the proton microprobe include values of: for garnet, DZr (0.4-0.7), DSr (<0.02-0.04) and DY (>2.5-9); for clinopyroxene, DZr (0.10), DSr (0.06) and DY (0.9); and for orthopyroxene DZr (0.18), DSr (0.04) and DY (0.18). These were determined for natural element abundance levels, and are consistent with data obtained for artificially enriched compositions, verifying Henry's law behaviour and the valid application of these data to geochemical modelling of natural systems.