Geochemical and isotopic data are presented for a suite of Palaeocene picrites, picrodolerites and crinanites from the Trotternish sills, northern Skye. The whole-rock geochemistry of the Trotternish sills is comparable with the overlying Skye Main Lava Series, although the sills are relatively Mg-rich (up to 30 wt% MgO) due to abundant cumulus olivine. Rare-earth element patterns exhibit overall LREE enrichment, relative to chondrite, and suggest an alkali basalt parental magma. Incompatible element patterns show Ba and K enrichment, relative to Nb, Th and U. Sr, Nd and Pb isotope ratios indicate that crinanites, which are the most fractionated magmas, are the most contaminated. Isotope ratios suggest up to 22% amphibolite-facies Lewisian gneiss may have been assimilated during fractionation in the upper crust, prior to multi-phase intrusion as sills. This is in contrast to the Skye Main Lava Series, which show evidence of crustal contamination by granulite-facies material in the lower crust, but is comparable with the Skye Preshal Mhor basalts and the Rhum intrusion. The Trotternish sills and the overlying plateau lavas therefore have a more complicated genetic relationship than was previously supposed.