The eruption of Mt. Pinatubo on the 12 th June 1991 affected the atmosphere in the tropics (20 S - 20 N) by stratospheric heating and by a change of the heterogeneous chemistry, due to a large sulphate aerosol load. We use the EMAC model, a numerical chemistry climate model to study the impact of the eruption on the atmosphere. The sulphate aerosols are prescribed and based on satellite observations. Three different simulations were performed, namely VOL including the full volcanic perturbation, NOVOL omitting volcanic aerosols and CVOL neglecting the heating induced by volcanic aerosols. The differences between the simulations separate the temperature effect from the pure chemical effect in the stratosphere. The maximum of the volcanically induced heating is reached 4months after the eruption at 50 hPa with 4 K. The total heating is composed of the radiative heating by stratospheric aerosols and by a change of the chemical composition. Neglecting the radiative heating by aerosols, the pure chemical effect heats the stratosphere at 10 hPa by 0.4 K and cools the lower stratosphere by 0.4 K. The total ozone column is reduced by 14 DU (6 %) within 3 months after the eruption lasting until the end of 1991. The contribution to this reduction arises primarily from the stratospheric heating by volcanic aerosols at the 20 to 50 hPa levels with 0.6 ppmv, which is 10 %. This volcanic heating increases the vertical ascent and displaces the O3 maximum to higher altitudes. Also, the temperature increase accelerates the heterogeneous reaction rates. The pure chemical effect shows a slight increase of ozone in the first year after the eruption and a decrease in the following years. Ozone increases at 10 hPa by 0.7 ppmv (4 %). The increased aerosol surface accelerates the heterogeneous reactions, so that more NOx is converted into HNO3. As a result the NOx cycle to deplete ozone slows down, and the ClOx, HOx and Ox cycle partly compensate the ozone depletion.