This work investigates the effect of a set of operating variables, including irradiation time, irradiation power, dosage of NaCl, settling time, pH, and the initial oil concentration, on the separation efficiency in the treatment of an oil in water (O/W) type cutting oil emulsion by microwave assisted demulsification. As a result of a series of batch demulsification tests a set of optimum operating conditions was found, consisting of 2 min of microwave irradiation at 280 W, the addition of 14 g/L of NaCl, 1 h settling time, and at a pH of 9.5. A separation efficiency of 93.8% was obtained with these conditions for 50mL of cutting oil emulsion with an initial oil content of 10 g/L. In addition, data from these tests were treated by a stepwise-regression method which results in a multi-variable equation. This empirical equation was able to describe the separation efficiency fairly well, after excluding those with a separation efficiency less than 40% and temperatures higher than the boiling point. Our principal component analysis (PCA) extracted four principal components with 78% of the total variance explained.