Organic Rankine cycles are a promising technology to convert waste heat energy into usable mechanical or electric power, giving them the potential to reduce fossil fuel emissions generated by traditional energy generation. The heat exchangers of these devices are of particular interest, as maximizing energy extraction from these free heat sources will increase net electrical power output. For this project I created an algorithm to predict the effects of mixture working fluids on the evaporator performance of an organic Rankine cycle generator for a wide range of waste heat source temperatures. This algorithm combines empirically derived heat exchanger performance parameters with the Lemmon and Jacobsen equations of state for mixtures of refrigerants to calculate the overall heat transfer coefficient (the UA value) for the specified entry conditions, allowing for outlet temperatures and net heat transfer to be predicted. Data was collected on a 10” x 20” x 40 plate flat plate heat exchanger using cool and warm water at various flow rates. Additional data was provided by Ener-G-Rotors from their refrigerant test bed. Parameters that can be varied within the algorithm are the mass flow rates and inlet temperatures of the heat source and refrigerant, as well as the composition of the refrigerant working fluid. This variability will assist in future system adaptations to new waste heat conditions that could be utilized by organic Rankine cycle technology.