In the literature of contingent valuation, a rights-based system of environmental ethics claiming that natural objects have absolute rights, has frequently been regarded as the main reason for incommensurability, i.e. for citizens’ inability to find a common measure according to which all values could be ranked. In a study of 2400 Finns aged between 18 and 70, we tested whether a respondent's commitment to guaranteeing private property rights could be a reason for incommensurability beyond the respondent's possible commitment to absolute nature rights. It was found that incommensurability, modelled with lexicographic preferences, was attributable more often to private property rights than to nature rights. However, Finnish respondents who had lexicographic preferences for nature rights based their choice more often on an ethical judgement, whereas lexicographic preferences for property rights could rather be explained with an ambivalent preference construction. Lexicographic preferences for nature rights increased the willingness to pay for conservation, while lexicographic preferences for property rights decreased it. The result, which was predicted by the theory, supported the validity of incommensurability measurement. The study therefore indicates that several reasons for incommensurable preferences may exist and that it is possible to measure these reasons in contingent valuation surveys in order to judge the validity of the welfare measures in environmental policy decision-making.