This study analyzes the influence of the institutional environment on firms' capital structure in the European Union (EU). Unlike other research, we focus on private firms and include data from all EU countries from 2010 to 2018. We split the sample into groups of small and large firms, which we consider financially constrained and unconstrained, respectively. Throughout the study, we posit that institutional effects on financing policies vary with size, with the effects being greater for constrained firms as they face more financing frictions. We also run regression models controlling for firm-level characteristics and relevant macroeconomic factors. Our findings reveal that a high-quality regulatory environment negatively affects firms' leverage, whereas social corruption has the opposite effect. Moreover, the efficiency of enforcing commercial contracts and developing the financial system positively influences firms' leverage.