High resolution electron microscopy (HREM) has been carried out on grain boundaries in dense (>90%) YBa2Cu3O7-δ, being both in the tetragonal and orthorhombic form. Grain boundaries were found to be predominantly parallel to (001) of one of the adjacent grains. No amorphous interlayer was observed in the grain boundaries. In samples sintered and annealed for several days deterioration due to intercalation of the crystal lattice near grain boundaries was observed. It is discussed that the low critical currents in sintered materials are caused by preferred grain growth in combination with the atomic structure at the grain boundary interface, and by the intercalation near the grain boundary.