The isotopic composition of fossil animal substances has performed an important role as an indicator of ecological signature for Cenozoic mammals over the last few decades. Recently, isotopic analysis of Cretaceous vertebrates has been used to reconstruct paleoecology, but the effect of diagenesis on such old materials has not been well understood. An evaluation of the effect of diagenesis on enamel from Cretaceous vertebrates is necessary to determine the reliability of isotopic paleoecology. I evaluated the extent of diagenetic effect on enamel from Cretaceous mosasaurs by using Fourier Transform Infrared Spectroscopy (FT-IR). Clear diagenetic alteration was not found in the enamel, showing Cretaceous material may retain pristine isotopic information.