We used a prospective design and structural equation modeling procedures to examine the processes by which people internalize the doing of new environmental behaviors. As predicted by self-determination theory (Deci & Ryan, 1985: Intrinsic Motivation and Self-determination in Human Behaviour, Plenum, New York; Deci & Ryan, 2000: Psychol. Inquiry 11, 227), participants who perceived the experimenter as autonomy-supportive evidenced greater internalized motivation regarding a set of self-selected environmental goals. Internalized motivation in turn predicted goal performance during the following week, which in turn predicted intentions to keep on striving after the study was over. Implications for the question of how to motivate people to engage in more environmentally responsible behavior are discussed.