Abstract In order to determine the structure of the 3 : 2 adduct (acetylene : isonitrile) obtained by cycloaddition reaction of dimethyl acetylenedicarboxylate with 2,6-dimethylphenylisonitrile and to elucidate the route of the reaction, a heavy atom derivative of this adduct was prepared and subjected to X-ray diffraction analysis. The crystal grown from acetone solution has the composition C36H34N2O12Br2·CH3COCH3 and belongs to monoclinic system with the lattice constants: a=34.13±0.04, b=14.66±0.02, c=8.10±0.01 Å, β=90.5±0.2°. The space group is P21⁄n containing four formula units in the cell. The structure was solved by the heavy atom method and refined by the block-matrix least-squares method for 1482 observed reflections allowing the anisotropic thermal motions for each atom. It has been shown that the chemical structure of the adduct is 1-(4-bromo-2,6-dimethylphenyl)-2-(4-bromo-2,6-dimethylphenyl)imino-1,2-dihydro-3,4,5,5,6,7-hexacarbomethoxy-5H-1-pyrindine.