The analysis of adverse selection problems in seller-buyer relationships has typically been based on the assumption that private information is uncerti…able, while in practice it may well be certi…able. If a buyer has certi…able private information, he can conceal evidence, but he cannot claim to have information for which he has no evidence, so he has fewer possibilities to misrepresent his information. Nevertheless, we …nd that the expected total surplus can be strictly smaller in the case of certi…able information than in the case of uncerti…able information. This …nding holds when the buyer may have private information with some exogenous probability as well as in the case of opportunistic information gathering, where the buyer can privately decide whether or not to acquire information for strategic reasons.