This paper presents the results of coupled neutral-ion photochemical model to demonstrate that gas-phase sulfur chemistry played a vital role in perturbations of the stratospheric ion composition following the Pinatubo eruption. Model calculations indicate that immediately after the eruption, the large amount of SO2 injected directly into the tropical stratosphere produced additional sulfuric acid vapor which increased the abundance of heavy negative ion family (called HSO4--core) by several orders of magnitude over the ambient. After 2 to 3 months, most of the sulfuric acid vapor gets condensed and converted to HSO4/H2O droplets (aerosols) and the density of ultrafine charged aerosol (which are driven by aerosol) also increases several folds (~1500%) in the tropics for 20--30 km. The perturbation in ionic species was evident even after one year of eruption (which attains a global coverage) but it finally settled down to its background level after about 2 years. ¿ 2000 American Geophysical Union