The chemical composition sensors on the Venera-4, -5, and -6 spacecraft are described. The mixing ratio by volume of carbon dioxide is determined to be 97 (+3, −4)%; nitrogen, less than 2%, oxygen, less than about 0.1%; and water vapor at the 25 °C temperature level, on the order of 10 mg/l, an amount large enough to imply that the clouds of Venus are composed of condensed water.