The article aims at presenting the influence of customary mores on the functioning of Islamic commercial banks. The paper discusses the most important customary mores, specifies the scope of their impact, and points out the differences between the principles of operation of Islamic banks and the banks based on the legal regulations binding in the market economy. Standards originating from the Koranic law are becoming increasingly common in a number of Muslim countries and constitute the sole basis for the principles of conducting business activity, what in turn helps the financial institutions retain their identity. Due attention should be paid to the financial institutions, which not only manage increasingly larger assets, but also intensify their cooperation. All this leads to a dualism of the financial markets. On the one hand, the Koranic legal standards have a significant impact on the mitigation of operational risk of financial institutions; on the other hand, however, they cause considerable departures from the market economy principles and blur the transparency of transactions concluded.