We consider collective decision problems where some agents have private information about alternatives and others don't. Voting takes place under strategy-proof rules. Prior to voting, informed agents may or may not disclose their private information, thus eventually influencing the preferences of those initially uninformed. We provide general conditions on the voting rules guaranteeing that informed agents will always be induced to disclose what they know. In particular, we apply this general result to environments where agent's preferences are restricted to be single-peaked or separable, and characterize the strategy-proof rules that ensure information disclosure in these settings.