The objectives of this study were to examine the ability of water and water-based metal salt additive systems to upgrade bitumen and determine the chemical mechanisms of observed reactions. These experiments were conducted on Peace River and Cold Lake bitumens and consisted of thermal treatments (375-415 degrees C) with and without water and with aqueous solutions of iron or ruthenium chlorides in the same temperature range. Little reaction was observed when either bitumen was heated dry or with water at 375 degrees C but significant conversion to insoluble products was observed at higher temperatures. Water had only a minimal effect in preventing formation of insolubles from Peace River bitumen but did result in reduction of the amount of insolubles produced in similar treatments of Cold Lake bitumen. The ruthenium/water additive produced oils with significantly lower sulfur content from both bitumens but at the expense of formation of high-sulfur-content insoluble products. However, addition of molecular hydrogen to experiments utilizing the ruthenium/water additive reduced the amount of insoluble products. The iron/water additive inhibited formation of insoluble products in comparison to experiments with or without water. Liquid products recovered from all experiment types except ones using molecular hydrogen contained more asphaltene than the unreacted bitumen but had much lower viscosities. In most cases, viscosities were low enough such that the bitumen would be suitable for direct pipeline transportation. GPC measurements indicated that the viscosity reductions were likely a consequence of reduction in asphaltene molecular size.