What gas makes up about 96% of Venus's atmosphere?

Carbon dioxide makes up about 96% of Venus's atmosphere. More precisely, measurements commonly give a composition of approximately 96.5% carbon dioxide and 3.5% nitrogen, with other gases present only in trace amounts.

That overwhelming concentration of carbon dioxide is the main reason Venus is the hottest planet in the Solar System. Carbon dioxide is a greenhouse gas: sunlight warms the surface, and the atmosphere absorbs and re-emits some of the outgoing infrared radiation, trapping heat. Venus's thick atmosphere amplifies this effect dramatically.

The atmosphere is not simply a dry version of Earth's air. Venus also contains sulfur dioxide, water vapor, carbon monoxide, and other trace compounds. High in the atmosphere, chemical reactions involving sulfur dioxide, carbon dioxide, and water vapor help produce clouds made largely of sulfuric-acid droplets. Those clouds completely hide the surface from ordinary optical observation.

A frequent mistake is to choose nitrogen because nitrogen dominates Earth's atmosphere at about 78%. Venus reverses that pattern: nitrogen is only a small minority there, while carbon dioxide is nearly the entire atmospheric mixture. Spacecraft including Venera 4 and later missions established the atmosphere's carbon-dioxide-rich nature.