Heavy oil fractions rich in asphaltenes were mixed with diluents containing from 0.6 to 25% aromatics, and the resulting blends subjected to batch precipitation experiments at 85C, and thermal fouling tests at surface temperatures in the range 230-310C. Deposit compositions were determined, and are compared with suspended asphaltene composition. When the heavy oils were blended with the most aromatic diluents, precipitation and fouling were negligible. As the aromaticity of the diluents was decreased, the extent of asphaltene precipitation and the fouling rates increased. The solubility parameter of the blends, mix, and the flocculation solubility parameter for asphaltenes, f, were determined from measurements of the asphaltene flocculation onset by titration with heptane at temperatures from 25 to 50C. Literature models predict no asphaltene precipitation (and presumably little fouling) will occur when [mix - f] 0. Both the amount of asphaltene precipitated in the batch experiments and the rate of thermal fouling decreased as the solubility parameter difference [mix - f] increased from negative to positive values. However, some precipitation and fouling were observed at the expected condition for mixture stability mix f. As the temperature of the flocculation titration was raised toward the bulk temperatures of the experiments, values of [mix - f] decreased, and the agreement of the data with prediction of the point for zero precipitation and fouling improved. For unstable oil blends, the solubility parameter provides a good predictive measure of the tendency for asphaltene precipitation and for heat exchanger fouling.