Absolute quantum yields for primary H atom formation (Phi(H)) were measured under collision-free conditions for the room-temperature gas-phase dissociation of acetylene (C2H2) after photoexcitation at 193.3 nm and at the H-atom Lyman-alpha wavelength (121.6 nm) by a pulsed laser photolysis (LP)-laser-induced fluorescence (LIF) 'pump-and-probe' technique. Using HCl and CH4 photolysis at 193.3 and 121.6 nm. respectively, as a reference, values of Phi(H) (193.3 nm) = 0.94 +/- 0.12 and Phi(H) (121.6 nm) = 1.04 +/- 0. 16 were obtained which demonstrate that for both photolysis wavelengths the H + C2H product channel dominates the primary C2H2 photochemistry. (C) 2002 Published by Elsevier Science B.V.