High critical current densities are necessary for major bulk applications of ceramic oxide superconductors. In this paper it is shown by high resolution Auger electron spectroscopy (AES) investigations that carbon impurities (originating from the carbonates) trapped in the microstructure of dense polycrystalline silver-sheathed Bi-Pb-Sr-Ca-Cu-O (BPSCCO) tape materials are present during sintering processes. The studies include measurements of the critical current density jc as a function of retained carbon concentration detected by AES techniques. In Ag/BPSCCO tapes with jc values varying from 1500 to 33 000 A cm⁻² at 77 K and zero field the retained carbon content indicates a drastic change from about 11 at.% to overall concentrations at the grain boundaries smaller than 0.8 at.% for maximum jc values. These results prove that a carbon content as low as possible is an essential requirement for obtaining ceramic oxide superconductors with relatively high critical current-carrying capacities.