We examine suspected molecular transformations of thermally treated Athabasca bitumen heavy vacuum gas oil (HVGO) by ultrahigh-resolution negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Liquid products from HVGOs; treated under an inert N-2 atmosphere at temperatures of 300, 325, 350, and 400 degrees C were each characterized by class (heteroatom content), type (double-bond equivalents = number of rings plus double bonds to carbon), and carbon number distribution. In addition, the inert N-2 sweep gas of the autoclave was collected, condensed, and analyzed. The total acid number (TAN) of the HVGO liquid products decreases with an increasing treatment temperature (from 4.13 at 300 degrees C to 1.46 at 400 degrees C), indicative of potential carboxylic acid decomposition. The highly abundant O-2 class contains species with DBE = 3; however, no compositional changes occur with increased treatment temperature. A bimodal DBE distribution is observed for the S1O2 class, suggesting two possible stable core structures. Only low relative abundance classes show slight changes with thermal treatment. Condensed nitrogen sweep gas obtained at 350 and 400 degrees C contains highly abundant O-2 species with DBE of 3 and but at lower carbon number. Similarly, the condenser product S1O2 classes display the same bimodal DBE distributions as the HVGO liquid products but with lower carbon number (similar to 18-27 for condenser versus similar to 25-35 for the liquid products). The similarity of the O-2 speciation in the HVGO liquid products after thermal treatment combined with the detailed analysis of the condenser products suggests that the gross decrease in total acid number (TAN) at higher temperature is due to global (class, DBE, and carbon number indiscriminant) decomposition of the naphthenic acids as well as a small contribution from the loss of the lower boiling, lower carbon number acids by simple distillation.