The isotopic abundance of strontium has been measured in representative rock-types from the Tertiary igneous suite in the Isle of Skye, north-west Scotland. The isotopic abundance of Sr 87 , expressed as the Sr 87 /Sr 86 ratio, for twelve basalts, dolerites, gabbros, and peridotites is 0.7058±0.0010. On the other hand, the average initial Sr 87 /Sr 86 ratio for one granite from the Eastern Redhills Complex and for three granites from the Western Redhills Complex is 0.7124±0.0015. The three principal members of the marscoite suite of the Western Redhills Centre, namely, ferrodiorite, porphyritic felsite, and marscoite, also have significantly higher initial Sr 87 /Sr 86 ratios than the basaltic rocks. It is concluded that the rocks with high initial ratios were derived from a source with significantly higher Rb/Sr ratio than that of the basaltic rocks. The hypothesis which is most in accord with the isotopic evidence is that the granitic rocks and the marscoite suite were produced by partial melting of ancient Lewisian rocks, which form the underlying basement at no great depth in this area. Rb—Sr (and K-Ar) age determinations were carried out on some representative rocks from the Eastern and Western Redhills Centres. It was not possible to establish a significant age difference between any of the rock units from either Centres. The mean age is 54±3 m.y. and suggests that intrusion of the granitic rocks of Skye occurred during a relatively short period of time within the Lower or Middle Eocene peroid.