In this work, machinability studies of AISI D2 steel on a die-sinking electrical discharge machine (EDM) using brass tool has been reported. A response surface methodology (RSM) is adopted to establish effect of various process parameters on important responses. Data envelopment analysis (DEA) is adopted to obtain relative efficiency for each experimental run treating them as decision making units (DMUs). Considering the relative efficiency as an equivalent response, the effect of each processing parameter is analysed and the optimal setting is reported. With the best combination of parameter setting, a thermo-physical model is developed to validate the optimal setting. Copyright © 2015 Inderscience Enterprises Ltd.