Viscosity and sulphur content are two important properties of heavy hydrocarbons for downstream processing. Both properties can be significantly reduced in value by processing in subcritical water. Subcritical water has advantages over steam recovery methods in terms of viscosity and sulphur reduction as well as energy consumption. In order to understand the mechanism of subcritical water treatment, the chemical structure of pre- and post-processed heavy hydrocarbons are compared to each other. For subcritical water processing, viscosity reduction coincides with a reduction in average molecular weight, average carbon chain length between branching points, and sulphur content, along with an increase in volatility and H2S production. This suggests that not only are C–C bonds cracked, but also C–S-(C) bonds. Subsequent comparison of subcritical water results with those obtained for pyrolysis treatment shows that the former is faster and more effective.