In work the peculiarities of crystallization of Аn+1(В,В')nО3n+1 (space group I4/mmm) and (Аn+2(В,В')n+1О3n+з3 (space group R-3c) complex oxides with cation relation А/ (B,B') = 3:2 are analyzed. It is established, that if A = Ca, Ba, Sr different structural modification crystallize depending on radius and oxidation degrees of В-positions cations. Increase of the average degree of oxidation of A, B, B'-cations as a result of replacement of an alkaline element for a rare earth element stabilizes the tetragonal structure. Obtained data allow predicting the results of synthesis and solid solutions limits. It is shown, that two modifications of a complex oxide Sr3ScCoO6+b exist. New complex oxides phases La 1,5Sr1,5CuMnO6.54La1,5Sr1,5Мn1,25Ni0,75O6.34 and LaSr2CoMnO7 with tetragonal structure are synthesized.