The structure and dielectric properties of (1-x)Pb(Sc2/3W1/3)O-3-(x)Pb(Zr/Ti)O-3 ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x less than or equal to 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(SC2/3W1/3)O-3 (PSW) can be described by a random-site model with one cation site occupied of (SC1/33+W2/36+). by Sc3+ and the other by a random distribution The ordering is destabilized in solid solutions of PSW with PbZrO3 (PSW-PZ), but stabilized by PbTiO3 in the (1-x)PSW-(x)PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW-PZ the temperature of the permittivity maximum (T-epsilon,T-max) increases linearly with x; however, for PSW-PT T-epsilon,T-max decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW-(0.25)PT; when the order parameter was reduced from similar to1.0 to similar to0.65, T-epsilon,T-max increased by more than 60degreesC.