Measurements of the fluorescence lifetimes and quantum yields of fluorescence from selected individual vibronic states of chloro- and bromo-acetylene are reported and the data analyzed in terms of the coupling of photon emission to the photodissociation reaction HCCX → HCC·(2Π)+ X·(2P) where X = Cl, Br. It is suggested that the reaction involves a slow internal conversion from the first excited singlet state to the ground electronic state, and that vibrational relaxation in the ground electronic state is sufficiently slow that randomization of the excitation energy does not occur before dissociation. Consequent competition between vibrational relaxation and resonance amongst quasi isolated groups of states leads to a departure from RRKM type behaviour. An unusual variation with energy of the nonradiative lifetimes of vibronic states is found. This observation is interpreted by taking note of the constraints imposed on the dissociation rate by the simultaneous conservation of energy, linear and angular momentum. The i.-r. and u.-v. spectra of chloro- and bromo-acetylene are re-investigated and differences between our findings and those of others are discussed in terms of the electronic structure of these molecules.