Stable carbon (delta C-13) and nitrogen (delta N-15) isotopic compositions of bone and dentine collagen extracted from museum specimens have been widely used to study the paleoecology of past populations. Due to possible systematic differences in stable isotope values between bone and dentine, dentine values need to be transformed into bone-collagen equivalent using a correction factor to allow comparisons between the two collagen sources. Here, we provide correction factors to transform dentine delta C-13 and delta N-15 values into bone-collagen equivalent for two toothed whales: narwhal and beluga. We sampled bone and dentine from the skulls of 11 narwhals and 26 belugas. In narwhals, dentine was sampled from tusk and embedded tooth; in belugas, dentine was sampled from tooth. delta C-13 and delta N-15 were measured, and intra-individual bone and dentine isotopic compositions were used to calculate correction factors for each species. We detected differences in delta C-13 and delta N-15. In both narwhals and belugas, we found lower average delta C-13 and delta N-15 in bone compared with dentine. The correction factors provided by the study enable the combined analysis of stable isotope data from bone and dentine in these species.