<p>Atmospheric particulate water is ubiquitous, affecting particle transport and uptake of gases. Yet, research on the effect of water on secondary organic aerosol (SOA) mass yields is not consistent. In this study the SOA mass yields of an <i>α</i>-pinene and m-xylene mixture, at a concentration of 60 μg m<sup>−3</sup>, was examined using an oxidation flow reactor operated at an atmospherically relevant RH of 60 % and a residence time of 160 s. Wet or dried ammonium sulphate and ammonium nitrate seed particles were used. By varying the amount of seed particle surface area, the underestimation of SOA formation induced by the short residence time in flow reactors was confirmed. Starting at a SOA mass concentration of ~ 5 μg m<sup>−3</sup>, the maximum yield increased a factor ~ 2 with dry seed particles, and on average a factor 3.2 with wet seed particles. Hence, wet particles increased the SOA mass yield by ~ 60 % compared to the dry experiment. Maximum yield in the reactor was achieved using a surface area concentration of ~ 1600 μm<sup>2</sup> cm<sup>−3</sup>. This corresponded to a condensational lifetime of 20 s for low volatile organics. The O : C ratio of SOA on wet ammonium sulphate was significantly higher than when using ammonium nitrate or dry ammonium sulphate seed particles, probably due to differences in heterogeneous chemistry.</p>