The way policy makers mobilize scientific knowledge in order to formulate environmental policies is important for understanding the developmental process of environmental policies. Some biodiversity conservation policies, such as those establishing the conservation units and laws on the regulation of land use in protected areas, were selected as objects of analysis. The aim was to see whether political decision makers are supported by scientific knowledge or not. Based on interviews with technical staff from governmental institutions, politicians and scientists, this study analyzed the way the knowledge is mobilized by policy makers concerning measures related to biodiversity conservation in the state of Rio de Janeiro (Brazil). We have concluded that environmental policy makers do not normally use the knowledge produced by scientific and academic institutions. Rather than being based on a systematic bibliographic research on environmental issues, the decisions are supported either by personal experience or by expert advice. The measures under analysis were not supported by evidence based on knowledge but motivated by political or economic interests. Paradoxically, policy makers consider themselves sufficiently well informed to make decisions concerning the policy to be implemented.