Microwave Limb Sounder (MLS) measurements of lower stratospheric ClO and gas-phase HNO3 are presented for the 1996--97 Arctic winter. The horizontal and vertical extent of enhanced ClO were smaller in 1997 than in 1996, as was the degree of enhancement. This is consistent with differences in the evolution and vertical structure of temperatures in the two years. Gas-phase HNO3 abundances, which increased in early winter due to diabatic descent, exhibited a decreasing trend from mid to late winter in all six Arctic winters observed by MLS, probably due to increasing HNO3 photolysis throughout this period. ¿ 1997 American Geophysical Union