This paper is in honor of Michael (Mike) T. Klein, and his contributions to the modeling of thermal conversion are highlighted within the context of this study. The question that was posed is whether thermal conversion models developed for conventional visbreaking (430-490 degrees C) are adequate for the description of visbreaking at lower temperatures? This topic is relevant to partial upgrading of bitumen by visbreaking at temperatures of <400 degrees C. Insights from thermal conversion performed at 100-430 degrees C were employed to revisit the description of the free radical chemistry and how temperature affected the relative importance of thermal reactions. With a decreasing temperature, the increasing contribution of reactions, such as molecule-induced homolysis and the presence of "persistent" free radical species in the feed, results in a higher free radical concentration than is predicted by initiation through thermally induced homolytic bond dissociation. With a decreasing temperature, transfer reactions are also increasing in relative importance. This affects propagation and termination reactions. One of the important consequences of the increased contribution of transfer reactions at lower temperatures is that the apparent activation energy of cracking is reduced. The threshold temperature below which conventional visbreaking models no longer provide an adequate description of conversion is 380-400 degrees C. Drawing on the differences that must be captured to model low-temperature thermal conversion, it was shown that the development work by Mike Klein provided a solid basis for such modeling. Of particular importance is the ability to incorporate reactive intermediates that include radical isomers and to model transfer reactions. Quantitative structure-reactivity relationships captured the essence of the chemistry that must be reflected to model low-temperature thermal conversion.