The superconducting transport properties (I–V, Ic –B and Ic –T) were correlated with a systematic characterization of the grain boundary microstructure, via TEM observation and local chemical analysis in STEM by EDX on YBa2 Cu3-xOy (D samples), YBa2 Cu3-xOy /Agx (S samples) and DyBa2 Cu3-xOy /1wt% Pt (0≤x≤0.4) ceramics. It was shown that in all samples studied the critical current I c is controlled by weak links at grain boundaries. The measurements of I c (T) indicated that in copper-deficient yttrium ceramics, Ag-doping leads to change of weak link character from superconductor-insulator-superconductor (SIS) to superconductor-normal metal-superconductor (SNS) type. This SNS behaviour seems to be a result of percolation path of supercurrent through clean boundaries with extremely narrow range (≅1 nm) of Ag segregation on them. Specific Ag precipitates of 2-5 nm size on or near clean boundaries were found in S samples for x=0.4. This is most likely to be a reason for the improvement of I c in these samples by a factor of 3. In copper-deficient dysprosium ceramics doped with Pt, Pt-doping leads to a substantial increase in Ic and pronounced ‘fish-tail’ effect in magnetic field at T≤77 K in non-stoichiometric composition with x=0.2. Better behaviour of I c in magnetic fields in these samples may be associated with the substantial increase in the proportion of clean boundaries and decrease in twin spacing.