Abundances of three incompatible elements (K, U, Th) have been determined by robotic spacecraft in five surface materials on Venus. We present these data normalized to terrestrial normal mid-ocean ridge basalt (NMORB) derived from the depleted mantle. Relative to NMORB, all of the Venus materials studied are enriched in all of these elements. The moderately enriched (Venera 9 and 10, Vega 1 and 2) basaltic rocks are similar to one another in the sense of their enrichment trends (UN>KN) but differ from the highly enriched Venera 8 material, where the trend is KN>UN. This difference implies that the enrichment pattern for the basaltic materials is not controlled by crystallization of an NMORB-like magma or by contamination of such a magma by highly enriched Venera 8 material within the crust, and the Venera 8 material cannot have evolved from the magma of any of the basaltic rocks. Our calculations show the K-U-Th pattern for any of the Venus rocks analyzed was not controlled by batch partial melting of primitive mantle. The Venera 8 material could be produced as a partial melt from eclogitic tholeiite, but none of the basaltic materials could be because the sense of their UN>KN trends is opposite to the enrichment trends in calculated models (KN>UN). The Venus basalts differ from fresh terrestrial rocks (NMORBs and oceanic island-arc volcanics) in having nearly constant K/U ratios, while terrestrial rocks have nearly constant Th/U ratios. This may suggest an unusual composition of mantle source(s) of the Venus basalts and/or unusual fractionation process(es) on that planet. ¿ 1999 American Geophysical Union