Solid solutions of the composition Ba${{}_{1-}{}_x{}_{-}}_y$(Mg, Ln)${}_x$${\text{Sr}}_y$TiO₃ ($x$ = 0.01; 0.025; 0.04; $y$ = 0.20; 0.50; 0.80; Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tu, Yb) were prepared by two-stage solid-phase synthesis followed by sintering using conventional ceramic technology. The influence of rare-earth elements on the microstructure of the prepared ceramic samples was investigated. It was found that regardless of the type of modifiers introduced, the grain landscape of the studied solid solutions with different amounts of SrTiO₃ is refined (in the initial system, the average grain size, $\bar{d}$, at $x$ = 0.20 is 6 $\mu$m; at $x$ = 0.50 is 4 $\mu$m; at $x$ = 0.80 is 18 $\mu$m) to crystallite sizes not exceeding (2-3) $\mu$m, and compacted. The using of mechanical activation procedures leads to an even greater decrease in the size and an increase in the density of ceramics. The increasing in the concentration of modifiers in each group (within the considered range of dopant variation) against the background of such a fine-grained structure has little effect on the dynamics of changes in $\bar{d}$. It is concluded that it is advisable to use the data obtained in the development of functional materials based on BST/(Mg, Ln) and devices with the participation of these compositions.Solid solutions of the composition Ba${{}_{1-}{}_x{}_{-}}_y$(Mg, Ln)${}_x$${\text{Sr}}_y$TiO₃ ($x$ = 0.01; 0.025; 0.04; $y$ = 0.20; 0.50; 0.80; Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tu, Yb) were prepared by two-stage solid-phase synthesis followed by sintering using conventional ceramic technology. The influence of rare-earth elements on the microstructure of the prepared ceramic samples was investigated. It was found that regardless of the type of modifiers introduced, the grain landscape of the studied solid solutions with different amounts of SrTiO₃ is refined (in the initial system, the average grain size, $\bar{d}$, at $x$ = 0.20 is 6 $\mu$m; at $x$ = 0.50 is 4 $\mu$m; at $x$ = 0.80 is 18 $\mu$m) to crystallite sizes not exceeding (2-3) $\mu$m, and compacted. The using of mechanical activation procedures leads to an even greater decrease in the size and an increase in the density of ceramics. The increasing in the concentration of modifiers in each group (within the considered range of dopant variation) against the background of such a fine-grained structure has little effect on the dynamics of changes in $\bar{d}$. It is concluded that it is advisable to use the data obtained in the development of functional materials based on BST/(Mg, Ln) and devices with the participation of these compositions.