Reaction of acetaldehyde cations with acetylene has been studied as a function of collision energy and acetaldehyde vibrational state. CH3CHO+ is state selected by resonance-enhanced multiphoton ionization, and scattered from C2H2 in a guided ion beam instrument. We have also calculated the structures and energetics of 11 different hydrogen-bonded, covalently bound, and coordination complexes, some of which are clearly intermediates in the reaction. From the product distribution, it is clear that some of most stable complexes do not participate in the reaction. The most exoergic product observed corresponds to CH3 elimination from a covalently bound complex, but this channel accounts for no more than a few percent of the total reaction signal. The dominant channel is hydrogen transfer (CH3CO++C2H3), which proceeds with > 73% efficiency at low energies, dropping to a near constant 30% at energies above 1 eV. Product recoil velocity distributions indicate that this channel is complex mediated at low energies, switching to direct scattering at high energies. The hydrogen transfer reaction is weakly affected by reactant vibration, including nu (3), the aldehyde CH stretch. Methyl elimination is strongly, but nonmode specifically inhibited by vibration.