Time-dependent neutron spectra in the center of an externally pulsed finite (7.16 × 25.02 × 25.4 cm3) H2O medium at 37.8°C were measured over an energy range 0.01 eV < E < 0.25 eV and for times extending from approximately zero to 100 µsec after the pulse. The phased mechanical chopper time-of-flight technique using the Rensselaer LINAC was combined with an on-line computer to obtain these measurements. A unique dual-disk neutron chopper was designed to follow the rapidly changing behavior of neutrons in a pulsed water medium with a chopper burst width of 3.4 µsec (FWHM). The neutron burst of this width was produced by the scissor-type chopping action of two over-lapping boron-10 loaded steel disks which rotated in the same direction at 11 250 rpm. Experimental results are presented in the form of the time behavior of neutrons at various energies (neutron life histories), time moments, neutron spectra at various times, the steady-state spectrum, as well as the energy and velocity moments. The average neutron velocity corresponding to the experimental spectrum at t = 23 µsec is within 2% of the average velocity corresponding to the asymptotic Maxwellian distribution. This indicates that the thermalization process is essentially complete in 23 µsec in the pulsed finite water medium that was studied. Due to spatial harmonics, the apparent exponential rate of decay determined from neutron life history curves for t > 30 µsec is only three-fourths of that corresponding to the fundamental decay constant for the medium under investigation. The results of a harmonic analysis based on cadmium-covered gold foil activations along the three axes of the water slab are in complete agreement with the neutron life histories at the medium center for 40 µsec < t < 100 µsec and with spatially dependent die-away measurements for 150 µsec < t < 900 µsec. Time-dependent diffusion theory calculations using a 78-group Haywood-II hydrogen kernel and a mass-16 free gas oxygen kernel have been performed with and without spatial harmonics. The inclusion of higher spatial modes in these calculations affects the amplitude of the time-dependent spectra by about 20% but changes the spectral shape at the high energy side by an increase of only 2 to 3%; the average energy increases by only about 1% for the time range 5 µsec < t < 60 µsec. The results of the theoretical calculations employing time-dependent diffusion theory indicate that the average velocity is within 2% of the asymptotic value after 16 µsec, less than the measured value. The agreement between experiment and calculation is however considerably better than had been obtained in the only previously published study; in that investigation time-dependent spectra measurements made on a large water medium indicated that the thermalization time was greater than 100 µsec.