The absolute rate coefficient of the reaction of the ethynyl radical with acetylene was measured using a pulsed laser photolysis/chemiluminescence (PLP/CL) technique in which HCC radicals were generated by excimer laser photodissociation of acetylene at 193 nm, and pseudo-first-order exponential decays of thermalized HCC were monitored in real time by the CH(A(2)Delta --> X(2)Pi) CL resulting from their reaction with O-2. The rate coefficient as determined using this single, absolute technique over the extended temperature range of 295 less than or equal to T (K) < 800 was found to exhibit no temperature dependence, the result being k(acetylene) = (1.3 +/- 0.1) x 10(-10) cm(3) molecule(-1) s(-1). B3LYP and CCSD(T)/6-311++G(d,p) calculations revealed that while the direct II-abstraction has a barrier of <approximate to>40 kJ mol(-1), the terminal C-addition is barrier-free giving the 2-ethynyl-vinyl radical (HC=C-CH=CH) which either dissociates directly into diacetylene (HC=C-C=CH + H) or first rearranges to 1-ethynyl-vinyl (HC=C-C=CH2) before undergoing a H-loss. In both these pathways, all intermediates and transition structures lie energetically well below the reactants and therefore both fragmentation pathways are open to thermal reactants. An additional C4H2 + H forming pathway through the vinylidene HC=CH-CH=C was identified; it is expected to contribute at higher flame temperatures. (C) 2000 Published by Elsevier Science B.V.