Abstract: Designing of the flow chart and fabrication of the on-line GC system used for determination of reaction products including ethylene, H2O, acetaldehyde, ethanol, ethylether and acetone in preparation of ethylene by catalytic dehydration of biological ethanol was described. It was required to keep the gas mixture sampled from the reaction tower running freely through the flow circuit of the GC analytical system for at least 60 min before performing the analysis. One mL of the gaseous sample was introduced. A stainless steel column (2 m,  4 mm) packed with GDX-104 was used as the stationary phase, and the column was kept at the temperature of 104 ℃. The 6 compounds mentioned above were separated completely under the prescribed condition through the column, giving a smooth baseline. Their contents were found by normalization of peak area. Values of RSD′s (n=5) found for the 6 compounds were 0.84%, 0.83%, 5.1%, 0.97%, 12% and 2.0%. Relative deviations between results acetaldehyde and ethylether found by the present method and by HPLC attained to relatively high values of -20.4% and -14.2% respectively. Reasons for sources of such errors were discussed.