Thermal conversion of bitumen by visbreaking is an important technology for partial upgrading of bitumen to pipeline transportable oil. The product from visbreaking of bitumen will likely spend a long time in storage or transport before being used, and due to cost considerations, postprocessing should be minimal. This study investigated the changes that take place over time during storage of visbroken bitumen (420 degrees C, 18-22 min) under nitrogen atmosphere in the dark. The study did not investigate phase instability, and it focused on chemical and physical changes taking place over a storage period of 20 weeks. With increasing storage time many properties of the visbroken liquid product changed; notably, refractive index, density, viscosity, microcarbon residue, and n-pentane insoluble content increased, while free radical content and aromatic hydrogen content decreased. The observed changes during storage are best explained by the formation of products with higher molecular mass. Although care was taken to limit the potential impact of oxidation, indirect evidence of the impact of oxidation due to dissolved oxygen was found during the initial 2 weeks of storage. Oxidation was not a prerequisite for free radical reaction during storage because the visbroken product already contained persistent free radicals, as is typical of high boiling petroleum fractions. Free radical termination reactions took place, but the extent of the changes in the properties with storage time was not consistent with simple radical-radical termination reactions as the only source of heavier products. Alkenes (olefins) formed during thermal conversion were likely involved in free radical propagation reactions by radical-alkene addition to form heavier products. The relationship between free radical content and storage instability, the nature of reactions taking place during storage, and the nature of the species in visbroken bitumen leading to storage instability were discussed. To reduce the formation of products with higher molecular mass during the storage of visbroken material, in addition to maintaining oxygen-free conditions, it is necessary to decrease the amount of free radicals and the amount of alkenes in the product.