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The influence of varying levels of water mixing ratio, <i>r</i>, during the formation of secondary organic aerosol (SOA) from the ozonolysis of α-pinene on the SOA hygroscopicity and volatility was investigated. The reaction proceeded and aerosols were generated in a mixing chamber and the hygroscopic characteristics of the SOA were determined with the Leipzig Aerosol Cloud Interaction Simulator (LACIS) and a Cloud Condensation Nuclei counter (CCNc). In parallel, a High-Resolution Time-of-Flight Aerodyne Aerosol Mass Spectrometer (HR-ToF-AMS) located downstream of a thermodenuder (TD) sampling from the mixing chamber, to collect mass spectra of particles from the volatile and less-volatile fractions of the SOA. Results showed that both hygroscopic growth and the volatile fraction of the SOA increased with increases in <i>r</i> inside the mixing chamber during SOA generation. An effective density of 1.40 g cm<sup>−3</sup> was observed for the generated SOA when the reaction proceeded with <i>r</i>>1 g kg<sup>−1</sup>. Changes in the concentrations of the fragment CO<sub>2</sub><sup>+</sup> and the sum of C<sub>x</sub>H<sub>y</sub>O<sub>z</sub><sup>+</sup> (short name CHO) and C<sub>x</sub>H<sub>y</sub><sup>+</sup> (short name CH) fragments as measured by the HR-ToF-AMS were used to estimate changes in the oxidation level of the SOA with reaction conditions, using the ratios CO<sub>2</sub><sup>+</sup> to CH and CHO to CH. Under humid conditions, both ratios increased, corresponding to the presence of more oxygenated functional groups (i.e., multifunctional carboxylic acids). This result is consistent with the α-pinene ozonolysis mechanisms which suggest that water interacts with the stabilized Criegee intermediate. The volatility and the hygroscopicity results show that SOA generation via ozonolysis of α-pinene in the presence of water vapour (<i>r</i><16.9 g kg<sup>−1</sup>) leads to the formation of more highly oxygenated compounds that are more hygroscopic and more volatile than compounds formed under dry conditions.