Eliciting sincere preferences for non-market goods remain a challenge due to the discrepency between hypothetical and real behavior and false zeros. The gap arises because people either overstate hypothetical values or understate real commitments or a combination of both. Herein we examine whether the traditional real-world institution of the solemn oath can improve preference elicitation. Applying the social psychology theory on the oath as a truth-telling-commitment device, we ask our bidders to swear on their honour to give honest answers prior to participating in an incentive-compatible second-price auction. The oath is an ancillary mechanism to commit bidders to bid sincerely in a second-price auction. Results from our induced valuation testbed treatments suggest that the oath-only auctions outperform all our other auctions (real and hypothetical). In our homegrown valuation treatments eliciting preferences for dolphin protection, the oath-only design induced people to treat as binding both their experimental budget constraint (i.e., lower values on the high end of the value distribution) and participation constraint (i.e., positive values in place of the zero bids used to opt-out of auction). Based on companion treatments, we show the oath works through an increase in the willingness to tell the truth, due to a strengthening of the intrinsic motivation to do so.