The O + CH3CCH reaction has been investigated in crossed molecular beam experiments at a collision energy of 38.7 kJ mol (1). Product angular and translational energy distributions have been measured for the H- and CH3-displacement channels and their relative branching ratio has been determined. The CH3-displacement channel is more important (by a factor of 3) than the H-displacement channel. This is not explained by a preference for the addition on the substituted acetylenic carbon atom, but rather by the different tendencies of the two addition intermediates CH3CCOH and CH3COCH to undergo intersystem crossing to the underlying singlet potential energy surface.