Major explosive volcanic eruptions inject gases and ash particles into the stratosphere, resulting in significant impacts on ozone and climate. Here we report on fast and sensitive in situ measurements of sulfur dioxide (SO2), hydrochloric acid (HCl), and nitric acid (HNO3) in an aged stratospheric plume of volcanic origin made by an ion trap chemical ionization mass spectrometer (ITCIMS) aboard the research aircraft Falcon. Other instruments on the Falcon measured reactive nitrogen (NOy), carbon monoxide (CO), and ozone (O3) as well as the aerosol composition. The plume most likely originated from the 7–8 August 2008 eruption of the Aleutian volcano Kasatochi and almost 3 months later, on 31 October 2008, was encountered by the aircraft during two missions over Europe at altitudes between 7000 and 11,600 m. Within the plume, SO2 mixing ratios of up to 0.51 ppbv were about a factor of 10 higher than in the background stratosphere (0.02–0.055 ppbv). The molar ratios HCl/O3 and HNO3/NOy tended to be elevated by up to 19%, and HNO3/O3 also showed a substantial enhancement inside the plume. From our measurements we infer an upper limit of the e‐folding lifetime of 62 days for SO2 conversion in the stratosphere, indicating that approximately 25% of the volcanic SO2 had not yet experienced OH‐induced conversion to sulfuric acid. Our measurements contribute to a better understanding of the formation of volcanic aerosol and of heterogeneous reactions taking place on these particles in the lower stratosphere.