First separation of the nuclear spin isomers of ethylene molecules ((CCH4)-C-13-C-12) and determination of the isomer conversion rate are reported. Experiment has revealed a linear increase of the conversion rate versus gas pressure equal to (5.2 +/- 0.8) . 10(-4) s(-1)/Torr. This indicates that the isomer conversion in (CCH4)-C-13-C-12 is governed by quantum relaxation.