Ejector systems are receiving considerable attention due to their simplicity, lower maintenance requirements, use of low grade heat, longer lifespan and low cost. In this paper an improved model to predict the performance of an ejector refrigeration system under both the critical and subcritical, modes of operation was developed and validated. The model predicts ejector performance more precisely compared to studies following the same modeling approach in the literature. Using the developed model, performances with environmentally benign refrigerants, including R1233zd(E), HF01336mzz(7), R1234ze(Z), R600, RE245fa2, and RE245fa2 as alternatives to R141b and R245fa were investigated. Furthermore, for ejector area ratios between 4.45 and 12.98, evaporator temperatures between 0 degrees C and 16 degrees C and condenser temperatures between 20 and 40 degrees C, the optimal performance of the ejector system was determined. Results show that for each refrigerant, higher area ratios give higher coefficients of performance, but require higher generator temperatures for better critical condensing temperatures. R600 showed the best performance followed by R12342e(Z) and R1233Zd(E) for the entire range of parameters considered. Results further show that there is an optimum generator temperature at each area ratio that maximizes performance. The optimal generator temperature increases as the area ratio and the condensing temperature increase. An alternative and more convenient approach to optimize ejector performance has been suggested in this work.