Following a review of the literature concerning droplet formation and the supersaturation of gas-vapour mixtures, an experimental test rig was designed and manufactured with which to study fog formation during rapid cooling and partial condensation. On completion a large number of experiments were performed using water vapour-air mixtures with a wide range of temperatures, partial vapour pressures and velocities. During each test, measurements of the droplet sizes and number concentrations were performed and recorded using a Laser Doppler Anemometer. The measurements were carried out at a number of points between the condenser walls, as the fog exited the experimental condenser. To complement the experimental program a theoretical study of fog formation within a partial condenser was completed. The resulting predictions of temperature drop along the condenser and bulk fog formation rates were used in conjunction with the experimental results and visual observations to present a clear description of the phenomena of fog formation within a partial condenser. Fog formation was shown to occur inside a condenser for a wide range of temperatures and relative humidities of the initial gas vapour mixture, though in many cases the vast majority of droplets were confined to a narrow region close to the condenser walls. In all the experiments conducted that led to a reasonable level of fog formation the total mass of liquid existing in the form of submicron droplets was insignificant (<< 1% of the total mass of fog). In general the extent of fog formation and the size of the droplets formed increased with increasing vapour pressure, coolant flow rate and condenser length, and with decreasing temperature and velocity. The presence of submicron droplets in the fog exiting the condenser was avoided completely by supersaturating the mixture by 15% or more prior to it entering the condenser.