The reaction dynamics of the dicarbon molecule With D3-methylacetylene, CD3CCH(X(1)A(1)), were investigated utilizing the crossed molecular beams approach. By combining the experimental results on the atomic hydrogen versus deuterium loss channels with electronic structure calculations, we could substantiate that the D-3 CCCCC(X(2)A) radical is neither formed on the singlet nor on the triplet surface. Secondly, the isotopic studies showed explicitly that on the triplet surface, only the 2,4-pentadiynyl-1 radical - detected via its isotopomer [C5HD2((XB1)-B-2); HCCCCCD2] - was synthesized. On the singlet surface, both the 2,4-pentadiynyl-1 [C5HD2(X-2 B-1), HCCCCCD2] and 1,4-pentadiynyl-3 radical [C5HD2(X-2 B-1) HCCCDCCD] can account for the experimental data. Published by Elsevier B.V.