Motivated by agency and stakeholder theories, this study aims to investigate the effect of corporate governance quality as measured by a single index on investment efficiency in the six Arab Gulf countries, commonly known as Gulf Cooperation Council (GCC) for the period 2015–2020. The study tries to develop a corporate governance quality model comprises 60 items under five main corporate governance elements of disclosure; responsibilities of the board; board effective composition; rights of shareholders and the role of stakeholders; and examines its effect on firms’ investment efficiency in 301 non-financial firms listed in six emerging capital markets in the GCC region. The findings reveal significant evidence that good corporate governance quality as a composite index enhances investment efficiency and mitigates both over- and under-investment. However, when the subcomponents of the corporate governance quality index were regressed individually on investment inefficiency, the findings were mixed. Our findings remain consistent when we control for potential endogeneity bias. The study has theoretically contributed to the corporate governance literature on corporate governance indices and quality by proposing a corporate governance quality model that considers all stakeholders. The practical implications of the study emphasize the significance of good company governance as a driver of investment efficiency; companies are expected to manage resources effectively, and regulators can implement regulations that enhance corporate governance standards in the GCC countries using the developed corporate governance quality model.

Purpose: This study aims to empirically examine the impact of internal corporate governance mechanisms (ICGM) related to the size of the board, board composition, CEO duality and audit committee independence as a single metric on a firm’s investment decisions.Design/methodology/approach: This study attempts to develop an internal corporate governance quality index comprising 10 items under four main ICGMs – size and independence of the board, CEO duality and audit committee independence – employing panel data analysis to investigate its impact on the investment decisions in 301 nonfinancial firms listed in six emerging capital markets in the Gulf Cooperation Council (GCC) member countries for the years 2015–2020. Data were extracted from sample companies' websites, stock markets, annual reports and Refinitiv database.Findings: This study provides convincing evidence that effective ICGMs minimize inefficient investment and ultimately boost investment efficiency. The findings remain consistent even after considering the potential endogeneity bias.Originality/value: This study provides empirical evidence on investment efficiency in the GCC region and emphasizes the importance of high-quality ICGMs in reducing inefficient investment. By examining the impact of ICGMs on investment inefficiencies, this study contributes to the corporate governance literature. The GCC region's unique economic and social contexts, with its growing economies, are considered to shed light on this issue.