This paper addresses the lack of empirical evaluation of the use of web based surveys in the context of contingent valuation surveys. We compare, using a case study, in-person interviews with web based surveys regarding response rate, information additivity effects and respondents' attitudes towards paying. The web based survey had a much lower response (5.1%) than the in-person interviewing (84%). We find the web based contingent valuation surveys to be neither more susceptible to information additivity effects nor more prone to zero protest responses. We conclude in favor of the use of web based surveys, namely in Portugal, where the number of Internet users is rapidly increasing, although further research efforts are required on their use.