Corporate social responsibility (CSR) researchers focus on the performance of the firms indexed in their field, and they reach different conclusions. This paper analyses if there are differences in financial performance of companies considered as highly socially responsible (those that compose the Spanish sustainability index FTSE4Good IBEX) as well as other IBEX companies not included in that index. We theoretically examine the relationship between a firm responsible behaviour and its financial performance. We also make an empirical study to analyse this relationship. The results show that the differential aspects between both groups of companies are market risk variables and debts and size variables.