Grain boundaries in Ag-sheathed (Bi/Pb) 2Sr 2Ca 2Cu 3O 10 tapes are investigated on the atomic scale, using a combination of Z-contrast imaging and electron energy loss spectroscopy in the scanning transmission electron microscope. The oxygen K and the copper L 2,3 ionization edges are studied to determine the copper valence at grain boundaries in two tapes exhibiting different critical current densities ( J c). The lower J c tape contains precipitates at high angle grain boundaries which appear to cause the formation of an extended non-stoichiometric zone, depleted in oxygen, and where the copper valence is less than or equal +1. In the higher J c tape, no reduction in copper valence is found at the high angle grain boundaries, indicating no depletion zone and suggesting improved transport properties. Thus, a correlation is found between the quality of the grain boundaries, the presence of other phases, and the overall J c of the tapes.