Thermal cracking of Athabasca bitumen at various reaction conditions with and without the presence of steam was investigated to determine if steam has a chemical influence on coking. The reactions were done in 15 mL microautoclave reactors and a 3 " diameter fluidized bed coking pilot unit over a range of reaction severity (350-530 degrees C, 10-60 min reaction time). The differences between reactions with and without steam were investigated by comparing elemental composition of the products and coke yield. The presence of steam decreased coke yield and decreased sulfur removal, and reduced the H/C ratio of the liquid products. Hydrogen exchange from steam to thermally cracked bitumen molecules was tested by doping water with deuterium oxide (D2O) and analyzing liquid and coke products by NMR and stable isotope mass spectrometry, respectively. Preferential deuteration of benzylic carbons was observed along with a trend of increasing deuterium transfer to liquids and coke as reaction severity increased. Free-radical, ionic, and physical mechanisms that can account for these experimental results are discussed.