Carbonate concretions are rather widespread throughout the Carpathians, being present in various strata from Lower Cretaceous to Miocene. Their mineral composition is strongly diversified, with prevalence of carbonates of iron, magnesium, calcium and manganese. They occur as lenses, bread-like forms and irregular layers, and their diameters range from some centimetres to some metres. The carbonate concretions of the Skole Unit are reported mainly from its shaly horizons and were found in the Spas, Dołhe, Ropianka, Variegated Shale, Hieroglyphic and Krosno Formations. Mineralogical investigations (optical microscopy and scanning electron microscopy, X-ray, DTA, chemical analyses) have allowed to distinguish several varieties of these concretions, typical for individual formations. The total of carbonates in the concretions, irrespective of the horizon in which they occur, range between 70 and 90%. Besides the carbonates, the concretions contain quartz and clay minerals, and some of them feldspars and pyrite. The concretions of the Spas Fm are sideroplesites (Szczepańska 1998). Their content of FeCO3 is the highest (50-60%) among all the concretions studied. The concretions of the Dołhe, Ropianka, Variegated Shale and Hieroglyphic Fm are built of various mineral mixtures of bi- or three-component carbonate solid solutions. High total contents of FeCO3 and MnCO3, ranging from about 50 to 75%, are their common feature. In the Krosno Fm the content of FeCO3 decreases (to several percents), while the contents of CaCO3 and MgCO3 increase (CaCO3 to more than 40%). These concretions are composed of ankerite and Fe-dolomite (Rajchel, Szczepańska 1997). The content of simple carbonates in concretions from individual horizons of the Skole Unit varies with the geological time. Similar relationships were reported by Narębski (1958) from the Western Carpathians. He distinguished four geochemical facies, considering the type of sediments and the type of concretions contained in them. In the Skole Unit, the Spas Fm can be included into Narębski's (1958) siderite-pyrite facies, the Ropianka Fm into the siderite facies, the Dołhe, Variegated Shale and Hieroglyphic Fm into the variegated facies, and the Krosno Fm into the dolomite-ankerite facies. The carbonate concretions studied are of early diagenetic origin.