This study examines the capital structure decisions of enterprises from a different perspective, specifically focuses on behavioral aspects. It investigates the behavioral tendencies of business managers, particularly their tendencies toward overconfidence in making capital structure decisions. To assess overconfidence, the median of the ratio of the sum of inventory and tangible fixed assets considered as collateral value of assets to total assets is utilized as a measurement method. In addition, the study analyzes other factors influincing capital structure decisions, such as collateral asset value, non-debt tax shield, growth, size, volatility, profitability, authenticity, leverage ratio and the imapct of COVID-19, while also explaining the effects of managerial behaviours. Using different models, 4 different models for the chemical sector and 4 different models for the cement sector, the effects of managers' overconfidence behaviours and other variables on capital structure decisions are explained with the dependent variables of KVBO (short-term debt ratio), UVBO (long-term debt ratio), KO (leverage ratio) and PD/DD (market-to-book ratio). In the study, the annual data of selected chemical and cement companies traded in Borsa Istanbul between 2011-2022 were utilised and panel data analysis method was used since the study data includes time and cross-sectional dimensions. The findings indicate that managerial overconfidence significantly influences capital structure decisions in both the chemical and cement sectors. Among the dependent variables evaluated in the chemical sector, the most pronounced effect was identified as a negative impact on UVBO. Conversely, in the cement sector, managerial overconfidence was found to exert a positive effect on the PD/DD value in capital structure decisions. The analysis section of the study further provides a comprehensive explanation of the detailed effects of managerial overconfidence on other variables influencing capital structure decisions.