A mathematical model is developed for evaluating wax deposition potential of petroleum reservoir fluids. Inputs to the model are viscosity-pressure data from VPT analysis. The model defines the region for predicting the potential of a given hydrocarbon system to deposit paraffin wax as it Bows up the well tubing and through the surface flow lines. Several applications of the model to Nigerian crude oil and gas condensate systems are reported. Viscosity prediction models for hydrocarbon systems at conditions below and above the bubble point pressure are also presented.