Response surface methodology (RSM) was used to evaluate the effects of processing parameters of a recent extraction process: the fast controlled pressure drop (DIC, "Detente Instantanee Controlee") on the extraction yield of rosemary essential oil. This process involves subjecting the rosemary leaves for a short period of time to a steam absolute pressure varying from 50 to 550kPa and then dropping the pressure instantaneously to a vacuum at 50kPa. In this study, the tested parameters were the processing pressure, the processing time and the initial moisture content of leaves before treatment and the responses were the global extraction yield and the extraction yield of the essential oil compounds namely alpha-pinene, camphene, 1,8-cineole, camphor, borneol and alpha-terpineol. The optimum processing conditions selected for the global extraction yield of essential oil obtained from response surface analysis were as follows: pressure level: 410kPa; moisture content: 0.40gH(2)O/g of dry material; processing time: 15.8 min. Under these conditions, the experimental extraction yield of essential oil was close to the predicted values calculated from the polynomial response surface model equation. The results showed that the processing pressure and processing time were the most significant parameters both on global extraction yield and the extraction yield of the different essential oil compounds. (c) 2004 Elsevier Ltd. All rights reserved.