The recently proposed [1] Th Hf Ta diagram is shown in the light of considerable additional data to be a sensitive indicator of the tectonic environment in which an unknown lava (basic of silicic) was erupted. With the presently available data it is now possible to distinguish calc-alkaline lavas from island arc tholeiites. Some of the fields of the different tectonic environments have been enlarged and the boundaries between the fields modified slightly, but the conclusions drawn by Wood et al. [1] are still supported by the data. It is not possible to discriminate between E-type MORB and tholeiitic within-plate basalts using this diagram, but when used in conjunction with the Zr Ti Y triangular diagram [23] (as originally suggested [1]) these magma types can be distinguished. The effects of different types of bulk lower and upper crustal contamination of a within-plate alkali basalt on the Th, Hf, Ta and radiogenic isotope concentrations of the residual liquids are calculated in detail. The ratios of Th Hf Ta are shown to be extremely sensitive to crustal contamination processes. Data published by Thompson and co-workers [2,5] for the Tertiary lavas of Skye, Scotland, are used to illustrate the complexity of crustal contamination and develop a petrogenic model for these lavas. These calculations suggest that the use of isotopic data alone to estimate the extent of crustal contamination a particular lava has suffered is fraught with errors and should be interpreted with care.