Using the laser photolysis-laser-induced fluorescence "pump-probe" technique, the gas-phase dissociation dynamics of HNCO((X) over tilde (1)A’)+h nu-->H+NCO and DNCO((X) over tilde (1)A’)+h nu-->D+NCO after photoexcitation at the Lyman-alpha wavelength were studied under collision-free conditions at room temperature. In the vacuum ultraviolet photodissociation experiments narrow band tunable Lyman-alpha laser radiation (lambda approximate to 121.4-121.6 nm) was used both to photodissociate the parent molecules and to detect the produced nascent H and D atom products via (2p P-2<--1s S-2) laser induced fluorescence. The following quantum yields Phi(H-D) for H-D atom formation were determined by a photolytic calibration method : Phi(H)=(0.62+/-0.15) and Phi(D)=(0.51+/-0.17). For HNCO and DNCO the measured H-D atom Doppler line shapes can be well described by a single Gaussian function, which corresponds to a statistical Maxwell-Boltzmann-like distribution of the translational energy. From the measured H and D atom Doppler profiles the average H and D atom kinetic energy was determined to be E-T(H)=(137+/-10) kJ/mol and E-T(D)=(115+/-4) kJ/mol, respectively. The average kinetic energies were found to be in reasonable agreement with results from simple statistical calculations in which it is assumed that H-D atoms are produced in combination with NCO in the ground electronic state ((X) over tilde (2) Pi), A dissociation mechanism is suggested in which H-D atom formation proceeds via a statistical unimolecular decay of a hot H-DNCO intermediate formed by a radiationless transition of the optically excited bound H-DNCO state to a lower-lying dissociative state.