An insturment for measuring the concentrations of cloud condensation nuclei as a function of nucleus critical supersaturation (S//c) is described. In the range 0. 1% less than equivalent to S//c less than equivalent to 0. 1%, the instrument is operated as a vertical flow thermal diffusion chamber. In the range of 0. 016% less than equivalent to S//c less than equivalent to 0. 1%, the chamber is operated at 100% humidity, and the equilibrium radii of the nuclei are measured with an optical counter. This paper presents a large body of data taken in the laboratory to evaluate the performance of the instrument. The data indicates that the instrument is internally consistent over a wide range of operating conditions.