An automatic instrument for the measurement of cloud condensation nucleus (CCN) concentrations utilizing a thermal gradient diffusion chamber and light scattering has been developed. The concentration of droplets (activated CCN) in an illuminated volume is determined by the measurement of the peak light intensity scattered at 45°. The CCN concentration is linearly related to the scattered light signal at a fixed supersaturation S, but the sensitivity exhibits an S0.55 dependence over a range of supersaturations. Calibration of the system against the photographic method verifies the linear dependence of scattered light on number concentration. The main features of the system are the automatic sampling, measurement and recording of CCN droplet concentration by means of scattered light with the capability of direct calibration by means of the photographic method. Results from four days of continuous hourly measurements of the CCN spectra show the influence of meteorological events on CCN. Included in the data are a frontal passage, fog formation and dissipation, and the occurrence of a nocturnal peak.