Because exchangeable cations may affect a shale's ability to transmit fluids and its response to some fluids used in fracturing or drilling, a study of the exchangeable cations in core samples from the Ohio Black Shale in Richland County, Ohio, and Perry County, Kentucky, was undertaken. Na/sup / is the cation usually associated with fluid problems, but it is generally low in the samples tested, so that problems related to ion-exchange are unlikely to occur. An unexpected analytical problem was encountered. A few percent of a carbonate mineral in a sample severely interferes with the cation determination by releasing Ca/sup /. Almost all of the samples apparently had sufficient dolomite or calcite to produce such an effect. Thus a basinwide study of exchangeable cations to test their effect on gas production, as was originally intended, is impossible. Also, the shale appears to be uniformly calcareous, perhaps having a small but ubiquitous amount of calcite cement.