No, that is incorrect. While carbon dioxide is a significant component of the atmospheres of both Mars and Venus, it is not the predominant gas on either planet.

Here's a breakdown:

* Venus: Venus's atmosphere is dominated by carbon dioxide, making up a whopping 96.5% of its atmosphere. The remaining 3.5% is mostly nitrogen.

* Mars: Mars's atmosphere is primarily composed of nitrogen, making up 95%. Carbon dioxide comes in second at 2.6%, with the remaining 2.4% being a mix of other gases like argon, oxygen, and water vapor.

So, while carbon dioxide is a key player in the atmospheres of both planets, Venus's atmosphere is overwhelmingly carbon dioxide, while Mars's atmosphere is dominated by nitrogen.