The multigroup Monte Carlo code KENO V.a and the 238- and 44-energy-group ENDF/B-V cross-section libraries were validated for 233U systems. Fifty-one critical experiments involving 233UO2(NO3)2, 233UO2F2, or 233U metal were selected for the validation. The H/233U ratios for the experiments range from 0 to 1986. Each experiment was modeled with KENO V.a, and the effective multiplication factor keff was calculated for each system using the 44- and 238-group ENDF/B-V, the 27- and 218-group ENDF/B-IV, and the 16-group Hansen-Roach cross-section libraries. The mean calculated keff for all experiments using the 44- and 238-group libraries is 1.0090 ± 0.0021 and 1.0064 ± 0.0020, respectively. For comparison, the mean calculated keff using the 27-, 218-, and 16-group libraries is 1.0142 ± 0.0038, 1.0125 ± 0.0038, and 0.9991 ± 0.0019, respectively. In general, an improvement exists in the agreement between the calculated keff’s and the experimental results (i.e., keff= 1.0) obtained with the newer ENDF/B-V libraries relative to ENDF/B-IV.