Isotopic analyses of 20 siderite spherulites, concretions and crystals from the continental Upper Cretaecous Whitemud and Edmonton Formations givedelta C 13 values between -2.7 and +12.8 per mil and delta O 18 values between -14.9 and -7.0 per mil on the PDB scale. The carbon isotopic composition of these siderites is controlled by biogenic carbon dioxide produced during diagenesis and therefore is not indicative of the depositional environment of the sediments. The constant oxygen isotopic composition of well preserved siderites likely reflects the freshwater milieu of their formation. Partially oxidized siderites are depleted in O 18 owing to secondary exchange reactions with "recent" percolating surface waters. Secondary changes of the carbon isotopic composition are less apparent because of the large spread of the primary delta C 13 values.