The asphaltene precipitation in a Chinese crude oil has been studied experimentally under normal-pressure and high-pressure conditions. The onset of asphaltene precipitation was detected by using light transmission method. Normal pentane and hexane were used as precipitating agents for studying the onset of asphaltene deposition in (asphaltene + toluene) and (degassed crude oil + toluene) mixtures under normal pressure. A total of eighteen sets of normal pressure data have been measured. For high-pressure gas-injected reservoir oil systems, the effects of injection-gas concentration, temperature, pressure, and the presence of a coexisting aqueous electrolyte (brine) phase on the amount of asphaltene deposition were studied and a total of seven sets of data have been measured. In the modeling part, a modified Hirschberg solubility model and a new solubility parameter correlation were proposed. The model parameters determined from low-pressure precipitation data have been successfully extended to high-pressure systems. Extensive tests vs. the data measured in this work and literature data show that the proposed model is capable of giving reasonable calculation results for most cases.