In the September 2004 edition of Climatic Change (N • 66), an editorial comment was published by Philip Fearnside, jointly with an article written by ourselves (Rosa et al., 2004) reporting on a broad-ranging project through which we measured CO 2 and CH 4 emissions at eleven Brazilian hydroelectric dams, from Itaipu, with the largest installed capacity in Southern Brazil, northwards to Tucuruí in Amazonia.This comment reflects a long-standing controversy over the estimates presented by Fearnside and the experimental results that we have obtained so far on greenhouse gases emissions by hydroelectric dams in Brazil (Rosa et al., 1996;Fearnside, 1996).The key issue in this latest comment by Fearnside is based on the fact that the water intake for the Tucuruí hydroelectric dam turbines is located at an average depth of 35 m, with a high methane concentration at a pressure of around three atmospheres 1 and an estimated temperature of 15 • C. This water pressure is suddenly eased after running through the turbines to the tailrace, flowing downstream from the hydroelectric dam, while it is drawn into the spillway at a depth of no more than twenty meters, far shallower than the water intake point for the turbines.The author of these comments uses the data published by us (Rosa et al., 1997) on the methane concentration measured by Tundisi 2 , who worked in cooperation with our research group. Fearnside commits a series of errors: he is confused about the transformation of units to express the Henry's constant, obtaining a value that is completely wrong; he errs when calculating the methane concentration in the water Climatic Change (2006) 75: 91-102