During the 1999/2000 Arctic winter SAGE III--Ozone Loss and Validation Experiment (SOLVE) campaign, high-resolution, in situ tracer data measured aboard the NASA ER-2 high-altitude aircraft revealed anomalous mixing events within the polar vortex. From January to March 2000 in the 350--500 K potential temperature range, we found mixing events during 15% of the flight time on average with significant maxima at potential temperatures of 450, 410, and 380 K. The events were spread throughout the vortex but showed a distinct minimum at 73¿ N and a peak at 85¿N equivalent latitude. About 60% of the observed mixing events were less than 13 km wide. Based on a case study of tracer-tracer relationships, an objective simple method is introduced to detect such events using the linear nitrous oxide (N2O):potential temperature relationship observed deep in the vortex. Rigorous analysis and supporting evidence from total water data corroborated the validity of the method. These results suggest mixing across the polar vortex edge occurred preferentially in layers at select altitudes in the Arctic winter 1999/2000.