Improved vie-filling compositions for producing conductive vias in circuitized ceramic substrates, particularly multi-layer substrates, without cracking and/or loss of hermetic sealing. The via-filling compositions comprise pastes containing a mixture of (a) ceramic and/or glass spheres of substantially- uniform diameter between about 0.5 and 6 .mu.m, (b) conductive metal particles or spheres having a maximum dimension or diameter between about 1/3 and 1/4 of the diameter of the ceramic and/or glass spheres, and (c) a binder vehicle. The formed conductive via bodies comprise a uniform conductive skeletal network of sintered metal particles densely packed within a uniform matrix of the co-sintered ceramic and/or glass spheres, which matrix is hermetically fused and integrated with ceramic layers forming the wall of the via in the ceramic circuit substrate.