A method for determination of very small concentrations of condensable vapors by measurement of the decrease in reflectivity of a cooled surface on which the vapor has been deposited is described. The method is applicable to mixtures of 2 components, one of which may be water. Use is made of the fact that a vapor will condense at a concentration below its vapor pressure in the presence of a second vapor whose liquid is soluble in that of the first. The most volatile component of the mixture is used in the pure state as a refrigerating agent whose evaporation cools the light‐reflecting surface to an accurately regulated temperature which produces condensation of the two components.An instrument embodying these principles is described which quantitatively measures triethylene glycol vapor in concentrations as low as 10−4mm Hg. The construction is simple and the operation continuous and automatic. The calibration of the instrument is presented. By proper adjustment, automatic compensation for changing atmospheric relative humidity can be secured so as to obtain response only to the degree of saturation of the glycol vapor in air.This device has been used to investigate condensation of vapors on atmospheric nuclei, and as a control instrument to regulate the concentration of triethylene glycol vapor employed as an aerial germicide for prevention of airborne disease transmission.