Thermal conversion of oilsands bitumen at 400 degrees C was investigated to gain a better understanding of temporal changes in liquid properties. The work approximated a mild thermal cracking (visbreaking) process with run-lengths extending into the coking region. Reaction progress could be divided into three main regimes: (I) stable visbreaking, (II) coking visbreaking, and (III) coking. In addition to observations anticipated from the literature, the work revealed aspects of the reaction progression that was not fully appreciated before. Stable visbreaking with minimal formation of coke had a productive period during which viscosity decreased, while asphaltenes content and gas yield were unchanged, followed by an unproductive period during which the viscosity, asphaltenes content, and gas yield all increased. After the onset of precipitation of solids, the solids (coke) yield increased and the asphaltenes content in the liquid decreased, but the viscosity increased. The origin of increased viscosity was not due to increased asphaltenes content but due mainly to free radical addition products that remained soluble in the bulk liquid. The insolubility of molecules in the liquid and meso-phase that ultimately led to precipitation as solids and coke formation at higher temperatures, appeared to be caused by addition reactions more than as result of asphaltenes and hydrogen-depletion. The investigation does not prove this conclusively, but if this interpretation is correct, the coking limit of stable visbreaking operation at 400 degrees C is not as result of a solubility constraint caused by hydrogen-depletion; instead, it is the result of free radical addition that manifests as a solubility constraint leading to hydrogen-depletion. In the reaction sequence, the importance of hydrogen disproportionation of alkyl cycloalkane functionality in the bitumen was highlighted to explain the major changes, including the onset of increased gas yield and increased viscosity that was attributed to free radical addition.