Liquid sulfuric acid aerosols at 190 K were generated in a cooled flowing system from the reaction between gas-phase sulfur trioxide and water vapour entrained in nitrogen. Infrared absorption spectra, identical to those previously presented in the literature, show the resulting aerosols to be ca. 60 wt.% sulfuric acid. Varying the H₂O partial pressure (pH₂O) changes the equilibrium concentration of the aerosol, such that decreasing pH₂O leads to spectral changes which can be assigned to a shift in the sulfate/bisulfate equilibrium in favour of the latter. Whilst adding Cl₂ did not result in any major changes to the aerosol infrared spectra, adding gaseous HCl resulted in marked changes. Vibrational mode analysis shows that the principal changes in the aerosol spectra are due to an increase in relative concentration of the sulfur-containing species as a result of a competition for water molecules needed for the ionisation of HCl in the aerosol particles. The sulfuric acid–bisulfate–sulfate equilibrium shifts significantly away from sulfate upon reaction with HCl, showing spectral changes which are characteristic of a transition from an amorphous liquid sulfate system towards one more characteristic of a bisulfate–sulfate system. The changes observed are partially reversible on the timescale of the experiment (4 min), although an undetermined proportion of the gas-phase HCl originally taken up remains with the aerosol in an ionised form.