Mixing diagrams of methane (CH4) concentration and stable isotopic ratio (&dgr;13C-CH4) were used to examine the fate of river-borne CH4 as it crosses a variety of estuaries: Columbia River (Oregon/Washington), Parker River (Massachusetts), Great Bay (New Hampshire), Kaneohe Bay (Hawaii), and Elkhorn Slough (California). Unlike the surface of the open ocean, these systems are not in near atmospheric equilibrium with respect to concentration of &dgr;13C-CH4 value. The range of observed CH4 concentrations and &dgr;13C-CH4 values were 33--440nM and -36 to -58 per mil, respectively, for the freshwater end-members for these systems, 12--330nM and -48 to -60 per mil for water at the mouths of the estuaries, and 1.6--6nM and -45 to -60 per mil for the seawater end-members. In the Kaneohe Bay estuary, CH4 concentration and &dgr;13C-CH4 displayed near-conservative behavior. In the Columbia River estuary, there was loss of riverine CH4 coupled with shifts to heavier isotopic values, apparently the result of in situ CH4 oxidation; this oxidation exhibited an apparent kinetic isotopic fractionation factor of 1.0042--1.012. In contrast, the other estuaries showed elevated concentrations and more negative &dgr;13C-CH4 values apparently resulting from inputs of biogenic CH4 from midestuary marshes and sediments. The upper reaches of all these systems were well out of equilibrium with the atmosphere on a concentration basis, indicating that they are atmospheric CH4 sources. However, these first &dgr;13C-CH4 measurements show that there is a wide range of isotopic variation in these waters, which indicates that it will be difficult to estimate the collective isotopic contribution of estuaries to the global methane budget. ¿ 1999 American Geophysical Union