From near-equatorial pitch angle measurements made with fine angular resolution on the CRRES and SCATHA satellites an investigation has been made of the location of the nightside outer boundary of the Earth's electron radiation belt and its dependence upon energy and mirror point altitude. The energy versus L dependence of the position of the trapping boundary is important for comparisons with models of the mechanisms to explain the loss of trapping at the outer edge of the radiation belt. Often the satellites passed through the trapping boundary very slowly, permitting detailed fine-scale measurements. With fine spatial resolution of the cutoff L values were frequently found to be almost independent of energy for E≤300 keV and were nearly independent of equatorial pitch angle up to 30¿. In the radiation belt near the outer edge sometimes the flux dropped to only intermediate values or to background level just for short times or equivalently narrow L intervals, suggesting time variations or the irregular onset of the mechanism(s) responsible for the trapping boundary. ¿ American Geophysical Union 1993