Sustainable intellectual capital is a promising starting-point for the incorporation of environmental aspects into the general management system of a firm. This paper examines how sustainable intellectual capital helps to overcome the shortcomings of conventional approaches to environmental management systems. It does this by extending the concept of sustainability from relational capital to the three pillars of intellectual capital: human, structural and relational intellectual capital. A multiple case study was carried out with Spanish firms characterized by their high level of environmental leadership. The findings indicated that sustainable human intellectual capital, environmental training, information and awareness sessions help in the accumulation and utilization of knowledge. As for sustainable structural intellectual capital, the environmental technology portfolio is improved and new environmental departments are created. Regarding sustainable relational intellectual capital, the firm–environment link is very important. In the primary and secondary sectors suppliers become involved to a greater extent in the environmental management process, whereas in the service sector it is the customer who becomes more involved. Copyright © 2009 John Wiley & Sons, Ltd and ERP Environment.