We assess the impact of environmental externalities on port folio decisions in a lab-in-the-field experiment on finance professionals and students. Participants are prone to accept lower returns for positive environmental impact but will not bear increased risk. They show a symmetric pro-environmental preferences depending on the sign of the externality. Finance professionals are more pro-environment than students, particularly regarding positive externalities, and less influenced by a ranking signal about environmental performance. Control tasks show that experimental measures of pro-social and environmental preferences have less influence on port folios than market practices for professionals but are significant predictors for students.