(Pb1-x-ySrxBay)(Zr-0.976-zTizNb0.024)O-3 solid solutions have been investigated to understand the relationship between structural changes caused by isovalent strontium and barium substitution on the A-site and dielectric and piezoelectric properties. As strontium and barium were substituted for lead, the zirconium:titanium (Zr:Ti) ratio was modified so that all compositions had an optimized piezoelectric coefficient (d(33)). The value of d(33) was at a maximum in the tetragonal phase near, but not at, the morphotropic-phase boundary (MPB). The real MPB was taken as the Zr:Ti ratio at which X-ray diffraction patterns appeared either pseudocubic or a mixture of rhombohedral and tetragonal. As strontium content increased, optimized d(33) also increased from 410 pC/N (x = 0) to 640 pC/N (x = 0.12), commensurate with a decrease in the paraelectric-to-ferroelectric phase transition temperature (T-C) from 350degreesC (x = 0) to 175degreesC (x = 0.12). However, for ceramics where x > 0.12, optimized d(33) decreased even though the phase-transition temperature was similar to150'C. Low strontium concentration ceramics (x = 0-0.08) contained 80 nm ferrroelectric domains typical of PZT, but high strontium concentration ceramics (x = 0.12-0.16) contained fine-scale domains (20 nm) in some regions of the microstructure. In addition, [110] pseudocubic electron diffraction patterns revealed super-lattice reflections at (1)/(2){hkl} positions associated with rotations of the octahedra in antiphase. Co-doping ceramics with strontium (x = 0.06) and barium (y = 0.06) resulted in the disappearance of the (1)/(2)[hkl) reflections. Optimized d(33) (similar to520 pC/N, T-C approximate to 205degreesC) for this composition was similar to that of ceramics where x = 0.08, y = 0, which had a T-C of similar to250degreesC.