The first step in the reaction of O(P-3) with HCCH is the formation of a complex, HCCHO which lives at least as long as a rotational period. The complex has two channels of dissociation, HCCO+H or (B1CH2)-B-3+CO. To find out more about this reaction the H and CO products were probed by laser-induced fluorescence. Earlier determinations that the branching ratio between the two channels is 1.4 are confirmed but it is shown that with DCCD the ratio is 0.9. This is explained by assuming that the 1,2 migration is hindered more severely by the deuteration than is the hydrogen atom dissociation. The average H atom translational energy agrees with the value of 13 kcal/mol found in Lee’s molecular beam study. The CO product of this very exothermic reaction is rotationally and vibrationally cold. The absence of rotational excitation implies a collinear transition state which is possible if the 1,2 H atom shift forms a ketene transiently in the B-3(1) State.