The vacuum residue prepared from Athabasca bitumen was heated at 380 degrees C for up to 5.5 h under vis-breaking conditions. The level of conversion was indicated by weight loss due to generation and removal of light ends from thermal cracking reactions. Classical molecular parameters that are commonly applied to indicate the level of thermal maturity of petroleum source rocks during burial maturation have been investigated toward indicating the extent of bitumen conversion under vis-breaking conditions. The molecular parameters known to monitor chemical reactions such as isomerization, carbon-carbon bond cracking, and aromatization showed strong correlations with the level of conversion during vis-breaking. The reaction mechanisms based on simple precursor-product relationships appear inherently more complex when quantitative molecular data are considered. For example, generation of Tm influences the behavior of Ts/(Ts + Tm), while changes in the triaromatic steroid parameter C-20/(C-20 + C28S + C28R) is controlled by the apparent relative stabilities of the C-28 VS C-20 compounds. The behaviors of a number of parameters based on saturated and aromatic hydrocarbons are mainly controlled by the relative stabilities of individual compounds. However, in view of the strong positive correlations shown by saturated and aromatic hydrocarbon parameters in-response to the extent of conversion, the parameters may be used to indicate maturity levels achieved under the vis-breaking conditions.