The properties of carbon, with respect to its ability to absorb and release hydrogen, are reviewedand applied to the interpretation of density behaviour in Ohmic-, NBI- and ICRF-heated tokamakdischarges. Based on the experimental observations, improvement of H-mode parameters in JET dueto reduced hydrogen re-emission from the walls is predicted with a numerical model. The particleremoval rates required for Q = 1 in JET are estimated.