Studying the relationship between global geographical diversification and firm performance in terms of corporate social responsibility, we posit that, because of the difference in institutional void, it makes a difference in this relationship whether an international firm is mainly diversified into developed countries (developed markets) or into emerging markets. Moreover, MNEs resource slacks may moderate the relationship differently. Testing empirically the effect of the diversification on two independent constructs, i.e., corporate social performance (CSP) and socially-irresponsible performance (CSiR), we obtain some interesting findings. (A) The diversification has a positive relationship with the CSP only in developed markets, and it has a negative relationship with CSP in emerging markets. (B) The diversification has a negative relationship with the CSiR only in developed markets, and it has no significant relationship in emerging markets. Finally, (C) MNEs slack resources moderate only the relationship between the diversification and CSP. Moreover, their moderating effects are all negative. However, MNEs slack resources do not moderate the relationship between the diversification and CSiR. We conclude with a discussion of the implications of the findings.