Experiments to isolate the essential oil from rosemary(Rosmarinus officinalisL.) were carried out using a new extraction process: “Détente Instantanée Contrôlée” (D.I.C) or “Controlled Instantaneous Pressure drop.” This process involves subjecting the rosemary leaves for a short period of time to a steam pressure, followed by an instantaneous pressure drop to a vacuum (about 15 mbar). This paper aims to assess the effect of processing pressure, processing time and initial moisture content of leaves before D.I.C. treatment on the extraction efficiency. The processing pressure varied from 0.5 to 5 bar, the processing time from 1 to 21 minutes and initial moisture content from 0.1 to 0.6 g H2O/g dm. It appears that a pressure of about 1 bar, and a treatment time less than 10 minutes are sufficient to extract more than 97% of the essential oil. The moisture content was not a critical parameter for the isolation efficiency. A stable yield and composition of the oil was obtained whatever the value of this parameter. The oil was also studied by quantitative and qualitative chemical analysis, the effect of modifying the operating conditions on the isolation of 10 components of the rosemary oil namely α-pinene, camphene, β-pinene, 1,8-cineole, camphor, borneol, terpinen-4-ol, α-terpineol, bornyl acetate and β-caryophyllene.