The fouling of heat exchangers by asphaltenes in petroleum crude oils is found to be caused not only by incompatible crude oil mixtures but also by mixtures that are nearly incompatible. It is reasoned that the tendency for asphaltenes to adsorb on heated metal surfaces increases as the oil mixture approaches compositions at which asphaltenes precipitate. Based upon data for mixtures of Forties and Souedie crude oils, this region of near-incompatibility occurs fir ratios of the solubility blending number to the insolubility number that are between 1.0 and 1.4. These numbers can be measured on individual oils using the oil compatibility model and tests and calculated for mixtures.