Laser induced fluorescence studies of hydrogen atom using four-wave mixing technique are reported for the photodissociation of H2O and D2O at Ly alpha (121.6 nm). The source of dissociating and probe radiation was one and the same (delay time <= 20 ns). The average translational energy of ejected hydrogen atoms (34.5 Kcal/mol) revealed that H2O + hv -> H + OH(X-2 Pi) and H2O + hv -> H-2 + O(D-1) were the main dissociation processes. The absolute quantum yield for H2O and D2O were Phi(H)(H2O) =1.39 +/- 0.09 and Phi(H)(D2O) = 1.21 +/- 0.20, respectively. The branching ratio of HDO, Phi(H)(HDO)/Phi(D)(HDO), was found to be 1.66 +/- 0.50, following the procedure developed by Shafer et al. [Shafer et al., J. Chem. Phys. 90 (1999) 6807]. (c) 2007 Elsevier B.V. All rights reserved.