The study aims at the separation of the undesired, light monoterpenes (MTs) of origanum oil from more valuable and heavier non-monoterpenes (NMTs). The scope of this work is the analysis of fundamental aspects of semi-batch, packed-column deterpenation process of origanum oil by dense CO2 at 70/85 bar pressure and 38/55 degreesC temperature levels. Effects of operating pressure, temperature zones, presence of packing, and entrainer (ethanol) use are investigated. GC/MS analyses of 14 major components constituting the bulk (86.4%) of the oil samples obtained from the top of the packed column in four different deterpenation time-cuts show that the resulting deterpenated product is rich in MTs (48-89% MTs, 52-11% NMTs). The oil remaining in the extraction vessel is rich in high-molecular-weight NMTs (15-24% MTs, 85-76% NMTs). Thus, the quality of origanum oil is improved as compared to the oil charged (36.44% MTs, 63.56% NMTs). Separation deteriorates with increasing deterpenation time. Enhancement of separation with packing is observed at the first time-cut. No significant entrainer effect (1.25, 2.5, and 5% ethanol in the charge) is noticed. Statistical analyses of the data and a response-surface analysis based on empirical models are presented. Suggested processing scheme that employs selective CO2 extraction and rectification has the potential for use in essential-oil refining technology.