[en] A slow, linear pressure rise in the drift ducts of the PLT neutral beam systems, and the resulting reionization losses of neutral beam power, are reported and examined. A model is developed for the reionization losses, based on the observed test stand pressure rise and an additional pressure rise due to the reionized power. Agreement between the model and the observed reionization losses is shown. Predictions are made of future reionization losses for the neutral beam injectors of PLT, PDX, Doublet III and TFTR. Finally, suggestions are made for research and development program directions and for machine design directions which could decrease neutral beam reionization losses