A method has been developed whereby the viscosity of a tar sand bitumen at any temperature can be calculated from nuclear magnetic resonance parameters. The method is semiempirical but is based upon some fundamental theoretical concepts for molecular mobility and intermolecular interactions. Using this method, the viscosities of three United States tar sand bitumens have been correlated to the weighted average spin-spin relaxation rates for the semiliquid, solidlike mobile, and solidlike rigid phases of the bitumens. The results indicate that bitumens with a high viscosity have a greater amount of solidlike rigid phase and lesser amounts of solidlike mobile and semiliquid phases than do the bitumens with low viscosity. It is also shown that the viscosity of a tar sand bitumen over a 100 degree temperature range can be determined from a single NMR experiment conducted near room temperature. 18 refs., 3 figs., 4 tabs.