Despite their apparent interest, private investors are surprisingly disengaged from sustainable investing, an observation that has received limited scholarly attention. This theory building study draws on the theory of planned behaviour to conceptualize the decision-making process of private investors towards sustainable investing. Findings from literature provide some insights but do not yield a comprehensive answer as to why private investors refrain from sustainable investing. Interviews with wealthy private investors led us to identify a generally high interest in sustainable investing and dominant barriers that prevent actual engagement. Barriers are the perception of high volatility within sustainable investments in combination with, first, a short investment time horizon and, second, recent financial losses. Third, we find that investment advisors withhold required information from their clients. We suggest a decision-making framework that facilitates a better understanding of the engagement of private investors in sustainable investing and outline avenues for future research and implications for practitioners.