This paper deals with the relevance of non use values in the evaluation of benefits from public action in environmental improvement. Cost Benefits Analysis of public investiments requires an evaluation of the demand for the involved public goods, which often has not a market price, and typically implies non use values. The Contingent Valuation (CV) is a well-established technique used to estimate non use values. This paper reports a CV study about the preservation of lagoon, beach and green areas in the island of S. Erasmo in the Venice Lagoon. Also, we investigate if non use values are taken in consideration when the respondents arrange his preferences in order to state a WTP. In compliance with it, we report an experimental approach to include non use values in the CV scenario