The effect of the substitution of BaZrO3 on the cation ordering in Ba(Mg1/3Ta2/3)O-3 was studied using TEM and X-ray and neutron diffraction. Almost no solubility of Zr was found in the 1:2 ordered, trigonal structure of the Ba(Mg1/3Ta2/3)O-3 end-member (P (3) over bar m 1), and a transformation to a 1:1 ordered, cubic (Fm (3) over bar m) phase with a = 2a(per) occurs for substitution levels between similar to 10 and 25 mol% BaZrO3. The structure of this Ba(beta’(1/2)beta "(1/2))O-3-type phase consists of two distinct octahedral sites, beta’ and beta ". The occupancies of the two cation positions, refined using the Rietveld method, were found to be consistent with a "random site" model in which beta " is occupied by Ta, and beta’ by a random distribution of the remaining cations. The homogeneity range of the 1:1 solid solutions predicted by this model, Ba{[Mg(2-y/3) Ta(2-2y/3) ZT(y)](1/2)[Ta](1/2)}O-3 (0.0 < y less than or equal to 0.5), is in excellent agreement with that observed experimentally.