The effect of the B-site cation chemistry and ordering on the dielectric properties of solid solutions in the (1-x)Pb(Mg-1/3 Nb-2/3)O-3-Pb(Sc1/2Nb1/2)O-3 (PMN-PSN) perovskite system was examined in samples with 0.1 less than or equal to x less than or equal to 0.9. Thermal annealing treatments were effective in inducing long-range B-site order in the samples within this compositional range. The well-ordered, large chemical domain ceramics exhibit relaxor behavior up to x = similar to0.5; for higher values of x, normal ferroelectric behavior was observed. For x less than or equal to 0.5 reductions in the chemical domain, size had no significant effect on the weak-field dielectric properties, but induced a transition to relaxor behavior for x > similar to0.6. The disordered PSN-rich samples undergo a spontaneous zero-field relaxor to ferroelectric transition similar to that reported previously for PSN. The field-dependent properties of compositions lying closest to the relaxor to ferroelectric crossover exhibited the highest sensitivity to alterations in the chemical order. The properties of this system are consistent with a "random site" description of the 1:1 ordered Pb(beta(1/2)' beta(1/2))O-3 structure with beta' = (Mg(2-2x)/3 Nb(1-x)/3Scx) and beta" = Nb.