This paper addresses the issue of rationality of responses in contingent valuation. We found a significant portion of respondents stated a positive willingness-to-pay for a project they perceived to be worthless. The irrational responses had a significant impact on the mean stated willingness-to-pay and the effect can not be removed by simply excluding the those observations from estimation. The dichotomous-choice elicitation format helps to mitigate the effect of the irrational responses, but can not completely solve the problem.