Earth Observing System Microwave Limb Sounder O3 and N2O are used to examine transport and chemical O3 loss in the unusually cold 2004--2005 Arctic winter. The vortex was dynamically active, with episodic mixing events throughout the winter; descent was the dominant transport process only through late January. Before the onset of lower stratospheric chemical loss, O3 was higher near the vortex edge than in the vortex core, causing different effects of mixing depending on the vortex region and time, either masking or mimicking chemical loss. O3 loss ceased by 10 March because of an early final warming. Rough estimates suggest maximum vortex-averaged O3 loss of 1.2--1.5 ppmv between 450 and 500 K, with up to ~2 ppmv loss in the outer vortex near 500 K. Despite record cold, chemical O3 loss was less in 2004--2005 than in previous cold Arctic winters.