During the last decade, the idea of sustainable investments hit the market. Investors both private and institutional started to supplement financial considerations with social and ecological ones. Meanwhile the supply of mutual funds in the green investment sector increased enormously. Currently in Europe about 300 mutual funds are available that are managed according to sustainability and social responsibility. Potential investors face the difficulty of keeping track of the various funds and choosing among them based on a reliable comparative assessment. This paper outlines the basic principles and methods on which such a comparative sustainability rating is based. The method was designed to be analogous to rating of the funds financially. The sustainability rating is based on assessment of the research processes in the fund management as well as investigation of the fund portfolio in terms of composition and sustainability performance. It should support investors in their investment choices by offering them a third party view. Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment.