The viscosity of normal alkanes, their mixtures, and true boiling point (TBP) fractions (C-6-C-19) Of four North Sea petroleum reservoir fluids have been measured by use of an automatic rolling ball microviscometer at 20 degrees C. The equipment is specially suited far samples of limited amount as only 120 to 2500 mu L is required depending on the viscosity range. The densities of the fluids were also determined. The accuracy of these measurements is ascertained and compared with literature data on n-alkane mixtures. The data reported for reservoir fluids includes molecular weights as well as density. Finally, generalized viscosity correlations for the C-6 to C-19 fractions are discussed.