In this paper we assessed the evaluation process of research groups? performance in the region of Valencia over the period 1998-2002, and the efficiency of such groups. The efficiency is assessed through the method of data envelopment analysis (DEA). Our findings show that publications in SCI/SSCI and the number of sexenios are the most determinant indicators of excellence in a research group. In terms of efficiency, we find that excellent research groups belonging to universities, in particular those in natural science, and R&D public organizations are the most efficiency.