We have compared composition changes of NO, NO2, H2O2, O3, N2O, HNO3, N2O5, HNO4, ClO, HOCl, and ClONO2 as observed by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat in the aftermath of the “Halloween” solar proton event (SPE) in late October 2003 at 25–0.01 hPa in the Northern Hemisphere (40–90° N) and simulations performed by the following atmospheric models: the Bremen 2-D model (B2dM) and Bremen 3-D Chemical Transport Model (B3dCTM), the Central Aerological Observatory (CAO) model, Fin- ROSE, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, the modeling tool for SOlar Climate Ozone Links studies (SOCOL and SOCOLi), and the Whole Atmosphere Community Climate Model (WACCM4). The large number of participating models allowed for an evaluation of the overall ability of atmospheric models to reproduce observed atmospheric perturbations generated by SPEs, particularly with respect to NOy and ozone changes. We have further assessed the meteorological conditions and th ... mehreir implications for the chemical response to the SPE in both the models and observations by comparing temperature and tracer (CH4 and CO) fields. Simulated SPE-induced ozone losses agree on average within 5% with the observations. Simulated NOy enhancements around 1 hPa, however, are typically 30% higher than indicated by the observations which are likely to be related to deficiencies in the used ionization rates, though other error sources related to the models’ atmospheric background state and/or transport schemes cannot be excluded. The analysis of the observed and modeled NOy partitioning in the aftermath of the SPE has demonstrated the need to implement additional ion chemistry (HNO3 formation via ion-ion recombination and water cluster ions) into the chemical schemes. An overestimation of observed H2O2 enhancements by all models hints at an underestimation of the OH/HO2 ratio in the upper polar stratosphere during the SPE. The analysis of chlorine species perturbations has shown that the encountered differences between models and observations, particularly the underestimation of observed ClONO2 enhancements, are related to a smaller availability of ClO in the polar night region already before the SPE. In general, the intercomparison has demonstrated that differences in the meteorology and/or initial state of the atmosphere in the simulations cause a relevant variability of the model results, even on a short timescale of only a few days.