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Abstract We determine the chemical changes associated with viscosity reduction when heavy oil is cracked in subcritical water. The viscosity reduction has a temperature threshold for onset of 290 °C—this suggests an enhanced acid cracking regime associated with the maximisation of water dissociation at these conditions aided by the already increased solubility. The mean molecular weight is reduced by nearly 50%. Oxygen and sulphur are reduced by about half of this—either by expelled gas effluent ($${\text{H}}_{2} {\text{S}}$$ H2S ) or by conversion into mono-aromatic base sulphur-containing structures. The amount of lower branched paraffins is increased.