A general equation is proposed for predicting the liquid viscosities of petroleum fractions based on a generalized pseudocompound method in which pure hydrocarbons and undefined hydrocarbon mixtures or petroleum fractions are treated as a hypothetical pure substance called pseudocompound which is characterized only by a boiling point and a density. The equation is tested by using the liquid viscosities of the petroleum fractions of typical American crude oils and crude oils from major oil producing areas. Good agreement between the predicted and experimental viscosities for the petroleum fractions is obtained.