The effects of lubricant miscibility and viscosity were evaluated by measuring the performance of an HFC-134a refrigeration system. The focus of this study is different from past studies which have been based on investigating system reliability in that is based on evaluating system efficiency, i.e., coefficient of performance;The vapor-compression refrigeration system used in this study contained a reciprocating compressor especially designed for the HFC-134 refrigerant and a direct expansion evaporator coil with a rated capacity of 10 kW (3 tons). With respect to the refrigerant side heat transfer surface, two types of evaporator coils were tested, namely the microfin-tube type and the smooth-tube type;The effects of refrigerant/lubricant miscibility were investigated by comparing the performance of a miscible polyol ester lubricant (POE) and an immiscible naphthenic mineral oil (MO) of similar viscosity grade. The coefficient of performance (COP) of the operating HFC-134a refrigeration system was found to be consistently higher, up to 5 percent, for the miscible POE lubricant compared to the immiscible mineral oil. The increased performance for the POE operation is attributed to an increased evaporation heat transfer rate for the POE lubricant. The results of other tested miscible lubricants also indicated superior system performance of miscible lubricants;The lubricant viscosity effects were investigated by comparing the HFC-134a system performance of two miscible POE lubricants of different viscosity grades, namely an ISO 22 and an ISO 32. The results show that both the COP and the capacity are generally higher, up to 3.5 percent, for the system employing the lower viscosity POE. Again, these performance differences were correlated to the differences in the evaporation heat transfer rate with the lower viscosity lubricant indicating a higher evaporation heat transfer rate;A partially miscible POE was also evaluated, and it resulted in a reduced performance compared to a fully miscible lubricant, and thus, it was concluded that partial miscibility in the evaporator has a negative effect on system performance;In contrast to the evaporator performance, the effects of lubricant type on compressor efficiencies were not substantial, as evident by the observed differences in both the isentropic and the volumetric efficiencies between tested lubricants being rarely larger than the respective uncertainties;It was observed that the effect of the lubricant type on performance was more pronounced for the microfin-tube coil. The microfin-tube coil outperformed the smooth-tube coil by up to 10 percent in terms of the overall heat transfer coefficient. In addition, the performance enhancement (i.e. the heat transfer enhancement for the microfin tube coil compared to the smooth tube coil) is more pronounced for the miscible POE lubricant;In summary, lubricant selection should not only be based on system reliability but also on the system performance and efficiency as evident by the fact that the use of miscible lubricants and a lower viscosity lubricant resulted in improved performance.