This paper aims to ascertain how company-specific factors influence the corporate ownership concentration of Portuguese firms. The paper employs several different regression techniques: Generalized Linear Model, Ordered Logit, 2 Stage Least Squares, Ordinary Least Squares, Truncated and Constrained regression. Additionally, to test the model's prediction power, it conducts an in and out-of-sample analysis and used joint-rolling window regressions and dependent variables intervals partition to test the robustness of the model under different sample restrictions. Firm size, profitability, the number of subsidiaries, and bank concentration are positive determinants of ownership concentration, while an opposite influence is found concerning auditor qualification and the board of directors' size. Significant implications are provided for the policymaking in countries where capital markets are underdeveloped, and concentrated ownership is common to help the regulator determining the power of controlling shareholders. This study enriches the literature on the determinants of corporate ownership, being the first study to approach non-public companies. It adds novelty by incorporating new company factors which are scarce in ownership studies.