Clouds profoundly influence weather and climate. The brightness and lifetime of clouds is determined by cloud droplet number concentration, in turn dictated by the number of available seed particles. The formation of cloud droplets on non-volatile atmospheric particles is well understood. However, fine particulate matter in the atmosphere ranges widely in volatility. Co-condensation of semi-volatile compounds with water increases a particle’s propensity for cloud droplet formation, with potential consequences for feedbacks between the terrestrial biosphere and climate. Here we systematically study cloud droplet formation, using a cloud parcel model extended to include co-condensation of semi-volatile organic compounds under a broad variety of realistic conditions. As an air parcel rises and cools, the concentration of organic vapour that it can hold declines. Thus, the simulated organic vapours become increasingly saturated as they ascend, and so condense on growing particles as they swell into cloud droplets. We show that condensation of increasingly volatile material adds to the soluble mass of these droplets and facilitates the uptake of additional water, which leads, in turn, to a substantial increase in the number of viable cloud droplets. We suggest that the co-condensation of semi-volatile organic compounds with water vapour has a substantial impact on the radiative properties of clouds.