The viscosity η (in centipoise) of a heavy oil sample is determined according to an equation of the form ln η η g = - C ⁢ ⁢ 1 * ( T - c ′ ⁢ 246 ) c ″ ⁢ 47.10 + ( T - c ′ ⁢ 246 ) , where T is the temperature of the heavy oil, T2LM is the logarithmic mean of the T2 distribution of the sample obtainable from nuclear magnetic resonance (NMR) measurements, c′=1.0±0.05, c″=1.0±0.04, ηg is the glass transition temperature viscosity of the heavy oil and a function of T2LM, and C1 is a variable which is a constant for the heavy oil and is a function of T2LM. Both C1 and ηg are considered functions of certain NMR values associated with the heavy oil sample, with ηg and C1 preferably estimated by empirically fitting data to the equations ln T2LM=a′+b′ ln ηg and ln T2LM=a″+b″C1, where a′, b′, a″ and b″ are constants.