A two-stage waste heat recovery device, adapted for installation as an integral part of an exhaust system of a vehicle equipped with an internal combustion engine for transferring thermal energy from waste heat from exhaust gasses contained within and passing through the device to a working fluid also passing through the device; comprising: a waste heat inlet configured to accept the exhaust gasses from the internal combustion engine, and a waste heat outlet and at least one waste heat containment chamber forming a part of the exhaust system and containing the exhaust gasses connected between the waste heat inlet and the waste heat outlet; a first evaporator, contained within the containment chamber, having a first working fluid inlet for receiving liquid state working fluid and a first outlet for output of a saturated vapor state of the working fluid; a second evaporator, also contained within the containment chamber, the second evaporator having a second inlet for receiving the working fluid from the first outlet of the first evaporator, the second evaporator having a second outlet for output of a superheated vapor state of the working fluid; a state separator, for separating vapor state working fluid and liquid state working fluid, connected between the respective first and second evaporators, the state separator having a separator inlet connected to the first outlet of the first evaporator, and a first separator outlet, for connecting to and providing liquid state working fluid to the first working fluid inlet of the first evaporator, and a second separator outlet for output of vapor state working fluid to the inlet of the second evaporator; wherein the working fluid is a mixture of a first fluid having a first boiling temperature at a given pressure, and a second fluid having a second boiling temperature at the given pressure; and, at least one butterfly valve positioned within the exhaust system at a downstream end of one of the evaporators, for controlling the rate of thermal contact of passing exhaust gasses with the first and second evaporators. a waste heat inlet configured to accept the exhaust gasses from the internal combustion engine, and a waste heat outlet and at least one waste heat containment chamber forming a part of the exhaust system and containing the exhaust gasses connected between the waste heat inlet and the waste heat outlet; a first evaporator, contained within the containment chamber, having a first working fluid inlet for receiving liquid state working fluid and a first outlet for output of a saturated vapor state of the working fluid; a second evaporator, also contained within the containment chamber, the second evaporator having a second inlet for receiving the working fluid from the first outlet of the first evaporator, the second evaporator having a second outlet for output of a superheated vapor state of the working fluid; a state separator, for separating vapor state working fluid and liquid state working fluid, connected between the respective first and second evaporators, the state separator having a separator inlet connected to the first outlet of the first evaporator, and a first separator outlet, for connecting to and providing liquid state working fluid to the first working fluid inlet of the first evaporator, and a second separator outlet for output of vapor state working fluid to the inlet of the second evaporator; wherein the working fluid is a mixture of a first fluid having a first boiling temperature at a given pressure, and a second fluid having a second boiling temperature at the given pressure; and, at least one butterfly valve positioned within the exhaust system at a downstream end of one of the evaporators, for controlling the rate of thermal contact of passing exhaust gasses with the first and second evaporators.