Abstract All previous theoretical investigations on grain boundary grooving are based on the assumption, that the grain boundary does not participate in the material transport. In the present paper it is shown theoretically and experimentally for the first time, that by removing this restriction one obtains completely different groove profiles in comparison to those predicted by the classical grooving theory. In couples of an Al-bicrystal contacting an In-A-melt and Cu-bicrystals in contact with a Bi Cu-melt instabilities of the grain boundary groove are observed. Deep, channel-like grooves at the grain boundary intersection with the solid-liquid interface appear after isothermal annealing. Diffusion of In into Al and Bi into Cu grain boundaries was detected. With a mathematical model it is shown, that the observed instabilities are result of the superposition of two processes: classical grooving due to curvature dependent morphological rearrangement of the grain boundary intersecting the solid-liquid interface and grain boundary diffusion of solute (In or Bi) occurring simultaneously. Profiles of the instabilities and the kinetics of the process are calculated.