Data from the CLAES on UARS indicate that a significant mechanism for production of HNO3 in the middle to upper stratosphere is missing from the chemical reaction set currently used by atmospheric models. Measured HNO3 in the polar vortex is strongly enhanced relative to the extra-vortex at 1200 K potential temperature (near 3 mbar) in January, 1992. The HNO3 vertical profile shows this enhancement forms a secondary altitude maximum from about 10 to 2 mbar (800--1500 K). A chemistry/transport model (CTM) simulation of this period produces no increase of HNO3 in the vortex near 3 mbar and no secondary maximum in the HNO3 profile. Furthermore, the CTM produces relatively high N2O5 in the vortex, with a vertical peak near 3 mbar, while both CLAES and ISAMS show a shallow minimum there. The implication of this comparison is that some unmodeled process is acting to enhance HNO3 and reduce N2O5 at high latitudes in the winter middle and upper stratosphere. Heterogeneous conversion of N2O5 to HNO3 on hydrated ion clusters is proposed as a possibility for the missing mechanism. ¿ American Geophysical Union 1995.