Assessing effects of diagenesis on oxygen isotope composition of bone is essential to its use in reconstructing habitats and lifestyles of ancient vertebrates. These effects are a matter of controversy, particularly in the case of extinct animals such as dinosaurs. To investigate the effects of diagenesis on isotopic composition of fossil bone, bone samples from both marine and terrestrial Campanian sediments from Alberta, Canada, have been analysed. The isotopic compositions of oxygen (δ18OSMOW) were determined in bones sampled from articulated skeletons of exclusively terrestrial animals recovered from the terrestrial Dinosaur Park Formation, and compared with bones from the marine Bearpaw Formation. The articulated skeleton of an exclusively terrestrial dinosaur (hadrosaur) found in marine sediments yielded similar δ18O values for both structural carbonate and phosphate fractions (mean δ18OSMOW values 22.6‰ and 16.9‰, respectively) in bone to marine reptiles (mosasaurs) recovered from the same locality (mean δ18OSMOW values 24.2‰ and 17.3‰, respectively). The isotopic composition of both skeletons recovered from marine sediments was significantly more positive than that of articulated hadrosaur skeletons recovered from contemporaneous terrestrial sediments (mean phosphate δ18OSMOW value 12.9‰), and outside the range of phosphate δ18OSMOW values previously reported for terrestrial dinosaur skeletons (c. 9–14‰). These data suggest that the isotopic composition of oxygen in the phosphate and structural carbonate ions in the bone apatite was altered during diagenesis and can be used for neither palaeoclimate nor physiological reconstruction.