Both 1:2 and 1:1 ordered structures form in the perovskite solid solutions of La-substituted BMT Ba1-xLax(Mg(1+x)/3-Ta(2-x)/3)O-3, sintered at 1600 degrees C. The 1:2 ordered structure exists in the composition range 0.0 less than or equal to x less than or equal to 0.12, while that of 1:1 ordered structure exists in a wider composition range 0.04 less than or equal to x less than or equal to 1.0. Two ordered phases coexist in 0.04 less than or equal to x less than or equal to 0.12. High-resolution micrographs indicate that 1:2 and 1:1 ordered domains coexist in one grain. The ordering parameter of 1:2 phase decreases with x, yet that of 1:1 phase increases with x. Both increase with soak time. Variations in ordering are discussed in terms of cation occupancy and crystal chemistry. The quality factor increases with x, reaches a maximum, then decreases with x. The dielectric constant increases with x first, and levels off.