Viscosity of heavy oil is determined in situ in a formation by making nuclear magnetic resonance (NMR) measurements in the formation, and then calculating viscosity according to an equation of the form T 2 LM = a + b ( .eta. T ) c , ##EQU00001## where T is the temperature of the heavy oil sample, .eta. is the viscosity, T.sub.2LM is the logarithmic mean of the T.sub.2 distribution spectrum of the sample, and a, b, and c are non-zero constants. Typically, constant b has a value between 5 and 7 and constant c has a value between -0.7 and -0.5.