Purpose The purpose of this study is to examine the effect of board composition on capital structure of a firm. Design/methodology/approach The paper uses data from firms listed in Nairobi Securities Exchange covering the period 2004-2012. Fixed effect regression model was estimated to test the effect of board composition on capital structure and how chief executive officer (CEO) tenure moderates the relationship. Findings The paper finds that board composition has important implications on capital structure decisions. Specifically, director independence is positively related to leverage, whereas CEO duality and tenure have negative and significant effect on leverage. In addition, the interaction effect of CEO tenure indicates that when CEOs have long tenure, the power of independent directors to influence capital structure decisions diminishes. Further, the study found that under long CEO tenure, long-tenure boards use less leverage in their capital structure. As expected, dual CEO with long tenure uses less leverage. Originality/value The study uses data from an emerging market, contrary to previous studies using data from developed markets, to test the relationship between board composition and leverage. Second, the paper tests the moderating effect of CEO tenure on board composition – leverage relationship based on the idea that entrenched CEO may influence the decision-making ability of directors, particularly capital structure decisions.