Picosecond transient absorption measurements and simulations of the hydrated electron geminate recombination dynamics are presented for 4 eV multiphoton ionization of liquid H2O and D2O. To within the accuracy of the data and simulation, the isotope effect can be accounted for in terms of the decrease in the diffusion coefficients for e(aq)(-), D3O+, and OD. and the e(aq)(-) + D3O+ --> D reaction rate in D2O.