Bitumen produced from oilsands deposits has a high viscosity, which presents a challenge to pipeline transport. Ways to reduce the viscosity at low incremental production cost are desirable. Thermal conversion of oilsands-derived bitumen at temperatures in the range of 150-300 degrees C was explored as a potential strategy for viscosity reduction. Viscosity increased compared to the bitumen feed following thermal treatment of bitumen at ISO and 200 degrees C but decreased following thermal treatment at 250 and 300 degrees C. At all temperatures studied, changes in the chemical and physical nature of the product were observed within 1 h of reaction time, and changes continued as reaction time was extended to a period of 8 h. Hydrogen transfer and methyl transfer were important reactions. These transfer reactions appeared to be concerted in nature and did not involve cracking to release free hydrogen or methyl radicals. In fact, thermal cracking was a minor reaction. The olefin content of the liquid was low, there was little gas-make, and the H2S concentration in the gaseous product was low. The n-pentane insoluble (asphaltenes) content of the liquid, with a few exceptions, increased during thermal conversion, but it was poorly correlated to viscosity. It is unlikely that the change in viscosity can be attributed to a single factor. The two most important factors appeared to be (i) the formation of heavier molecules that caused an increase in "excluded volume" with a concomitant increase in viscosity and slight decrease in density and (ii) a change in the phase behavior of the product due to chemical changes in the product.