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This paper presents a two-phase cooling method using the R134a refrigerant to dissipate the heat energy (loss) generated by power electronics (PEs), such as those associated with rectifiers, converters, and inverters for a specific application in hybrid-electric vehicles. The cooling method involves submerging PE devices in an R134a bath, which limits the junction temperature of PE devices while conserving weight and volume of the heat sink without sacrificing equipment reliability. First, experimental tests that included an extended soak for more than 850 days were performed on a submerged insulated gate bipolar transistor (IGBT) and gate-controller card to study dielectric characteristics, deterioration effects, and heat-flux capabilities of R134a. Results from these tests illustrate that R134a has high dielectric characteristics and no deterioration of electrical components. Second, experimental tests that included a simultaneous operation with a mock automotive air-conditioner (A/C) system were performed on the same IGBT and gate-controller card. Data extrapolation from these tests determined that a typical automotive A/C system has more than sufficient cooling capacity to cool a typical 30-kW traction inverter. Last, a discussion and simulation of active cooling of the IGBT junction layer with the R134a refrigerant is given. This technique will drastically increase the forward current ratings and reliability of the PE device. Index Terms--Power electronic (PE) cooling, thermal management, two-phase cooling.