Abstract: The invention provides a solvent-assisted method for mobilizing viscous heavy oil or bitumen in a reservoir under reservoir conditions without the need to adjust the temperature or pressure. The invention utilizes mixtures of hydrocarbon solvents such as ethane, propane and butane, which dissolve in oil and reduce its viscosity. Two or more solvents are mixed in such proportions that the dew point of the solvent mixture corresponds with reservoir temperature and pressure conditions. The solvent mixture, when injected into a reservoir, exists predominantly in the vapor phase, minimizing the solvent requirement. The invention can be practised in the context of paired injector and producer wells, or a single well cyclic system.