Thirty-two pairs of coexisting cherts and carbonates were analyzed for the O 18 /O 16 ratios. The relationships among the ratios of the different phases give some pertinent information on the origin and history of cherts. The O 18 /O 16 ratios of limestone and chert pairs appear to respond to diagenesis in a similar manner, both decreasing with time. Presumably this is a result of slow exchange with surface or subsurface waters. The O 18 /O 16 ratios of cherts interbedded with marine limestone appear to be due to consolidation in a "marine" environment. Data are reported for the C 13 /C 12 ratios of limestones and the O 18 /O 16 ratios for a few diatomites.