Selective photolysis of a mixture of acetylene isotopomers prepared in rovibrational states lying in the energy window of the second overtone of C-H stretches was performed. Tunable near infrared (NIR) photons prepared the isotopomers in initial states and ultraviolet photons both promoted them to the excited electronic trans-bent (A) over bar (1)A(u) state and tagged the H(D) photofragments via (2 + 1) resonantly enhanced multiphoton ionization. Photoacoustic spectroscopy monitored the initial state preparation and action spectroscopy simultaneously tagged the H(D) photoproducts yield as a function of the wavelength of the exciting NIR laser. The measurements and analysis indicate differing dissociation efficacies in C2H2 and C2HD revealing enhanced C-H bond cleavage in the former and hindered C-H(D) bond fission in the latter. The enhancement is a result of coincident resonances with the upper state leading to sharp structures in the photodissociation cross-section of CH2. (C) 2002 Elsevier Science B.V. All rights reserved.