In this article the basic types of capital characteristic for modern financial institutions were presented. Based on the Polish Financial Supervision Authority, the stability of funding sources in a group of commercial, cooperative banks and credit institutions setting up the Polish banking system in the years 2009-2014 was estimated. The conducted study showed that the main funding sources of commercial banks activity, cooperative banks in particular, are deposits of non-financial sector. The emission of debt commitments represents a very small share in total assets. In addition, the internal analysis of the structure of deposits of non-financial sector structure showed that the dominant source of capital are cash flows from the household sector. It should also emphasize that despite the reduction in central bank interest rates, the dynamics of increase in fundraising, is keeping an upward trend, which greatly reduces the instability of funding sources.