Semi-batch deterpenation of origanum oil (Origanum Munituflorum) by dense (sub/supercritical) CO2 was studied in an unpacked column with two temperature zones which were applied to column's upper and lower sections. Experimental results at combinations of 70 and 85 bar pressure and 38 and 55 degrees C temperature levels were reported. The results were discussed in terms of 14 major components, and the monoterpene (MT) and non-monoterpene (NMT) fractions of the oil. The oil was obtained from the top of the column at four different deterpenation time-cuts (integral samples of 0-15, 15-25, 25-35, and 35-45 min time intervals). In addition, the effects of the amount of oil feed charged to the extraction vessel, the solvent-to-oil ratio, and the presence of glass beads (0.5 cm) in the column as packing material were demonstrated. The best separation between the MTs and NMTs was achieved at 70 bar with 38 degrees C lower-section and 55 degrees C upper-section temperatures. At these conditions, the MTs are preferentially separated (with relative-distribution-ratio values greater than 2.0) from NMTs (with relative-distribution-ratio values less than 0.5), almost independent of the deterpenation time (flat maximum occurs in the 15-25 min deterpenation time-cut). It is concluded that any occurrence of single-phase state in the column deteriorates the effectiveness of the separation. Lower densities (higher temperatures) at the upper section of the column enhance the separation by causing internal reflux. Statistical analyses of the data and a response-surface analysis based on empirical models are presented. The suggested processing scheme that employs selective CO2 extraction and rectification has the potential to be used in essential-oil refining technology. (C) 2000 Elsevier Science B.V. All rights reserved.