The euro's share among the currency reserves held by central banks fell from 25.1 percent to 23.9 percent in 2012. Meanwhile, countries held more reserves in currencies of nations with strong economies, such as Australia and Canada.
Countries hold foreign currency reserves to help backstop their own currencies' value for trade and in case of a financial crisis.
The ECB said Tuesday there was comparatively less in euro-denominated bonds issued by companies, as borrowers found they got lower interest rates if they sold bonds in dollars.
The euro remains the second-most important reserve currency after the dollar, whose share was little changed at 61.9 percent.