Absolute partial cross sections for electron-impact ionization of H2O and D2O are reported for electron energies from threshold to 1000 eV. Data are presented for the production of H2O+ + OH+ + O+, O2+, H-2(+) and H+ from H2O and for the production of D2O+, OD+, O+, O2+, D-2(+), and D+ from D2O. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ions are completely collected. The overall uncertainty in the absolute cross section values is +/- 4.5% for singly charged parent ions and is slightly greater for fragment ions. The cross sections for H2O and D2O are found to be the same to within experimental uncertainties except for the H, cross section which is approximately a factor of 2 greater than the D-2(+) cross section. Previous results are compared to the present measurements.