Sr isochron plots distinguish three different suites of granitic rocks in the vicinity of Okanagan Lake. Voluminous granites sampled over a large area east and west of the lake have scattered 87Sr/86Sr isotopic ratios (0.706–0.710) that indicate either a Paleozoic age (possibly as old as Devonian) or heterogeneous and high initial ratios in Mesozoic granites. The second suite, a composite from localities near Whiterocks Mountain, gives an isochron date of 150 ± 5 Ma with an initial 87Sr/86Sr ratio of 0.7039 ± 0.00015. The third suite, from the Whiteman Creek stock and granites along the shores of Okanagan Lake and Wood Lake, gives an isochron date of 59 ± 2 Ma with initial ratio 0.70515 ± 0.00015. The secular increase in initial ratios reflects crustal thickening and evolution. These new dates are consistent with Paleozoic (circa 400 Ma), Mesozoic (180 and 150–160 Ma), and Cenozoic (50 Ma) igneous episodes previously recognized in the border zone between the Intermontane and Omineca belts in southern British Columbia.