To identify pathways to lower environmental impacts, this research examined the motivation and antecedents of motivation (autonomy, relatedness, competence), of environmentally active people. Previous research suggests that people with more self-determined motivation for pro-environmental behavior (PEB) should carry out more PEBs, and have lower environmental impacts, than people whose motivation is more externally regulated. Path analysis in Sample 1 (N = 261) confirmed that self-determined motivation was positively related to both easy and difficult PEB. The more participants judged that their needs for autonomy and relatedness were met in relation to performing PEB, the more self-determined their motivation. Higher perceived relatedness was also directly related to reporting more engagement in difficult PEB. Perceived competence was not related to self-determined motivation or PEB. The pattern of results was largely supported when re-tested with a sample (N = 320) who completed a "carbon footprint" measure of environmental impact as well as the questionnaire completed by Sample 1. In this sample, autonomy, relatedness, "and" competence were related to self-determined motivation. The research is the first to our knowledge to examine and find a relationship between higher self-determined motivation and lower self-reported environmental impact. These findings point to new approaches to increasing PEB.