The products of the catalytic reduction ethyne by nitrogenase in D~2O were studied by ^1H NMR. In order to assign the ^1H NMR spectrum, the theoretical spectra of C~2H~2D~2 were calculated using group symmetry. The final eigenfunctions,eigenvalues,allowed transitions and intensities of C~2H~2D~2 and C~2H~2D were obtained.It can be concluded that the products comprised mainly of cis-C~2H~2D~2.There was also some C~2H~3D. ^1H chemical shift in C~2H~3D moved by ca.4. 0Hz to the upfield compared with that in C~2H~4. The upfield shift in C~2H~2D~2 was about twice as large as that in C~2H~3D. The theorical results are in good agreement with experimental ones.

Key words: ACETYLENE, CATALYTIC REACTION, NITROGENASE, HEAVY WATER, CHEMICAL SHIFT, PROTON MAGNETIC RESONANCE SPECTROMETRY