This paper examines the effect of state ownership on the effective tax rates of Spanish companies. Using information regarding 3169 companies during the period of 2008–2014, we show that there are significant differences between the tax burdens of non-state-owned enterprises (NSOEs) and state-owned enterprises (SOEs), with the effective tax rates of private ownership companies being higher than those of state-owned firms. Company features, such as size, leverage, research and development investment, profitability, firm age, foreign operations, and auditing determine the tax burden of private ownership firms. That of state-owned companies, however, is affected only by leverage and capital intensity. For both SOEs and NSOEs, the tax burden is lower when they are taxed under the Spanish special taxation regime for small- and medium-sized enterprises. In short, company characteristics are more important in private ownership firms, in which almost all the variables considered have certain repercussions. This result may be because private ownership companies devote more resources to tax avoidance, and their fiscal strategy may determine their economic and financial structure. However, SOEs present significantly lower effective tax rates than NSOEs, probably because of the tax incentives that the law provides for them to support their sustainability.