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Abstract. The phase equilibria of the La-Me-Co-O systems (Me = Ca, Sr and Ba) were studied in air at 1100 ~ Two types of solid solution of general composition LavxMexCoO~_8 and (Lal.y Mey)2CoO 4 were found to exist in the systems. The limiting composition of Lal_xMexCoO3 ~ lies at x = 0.8 for Me = Sr, Ba and between 0.3-0.5 for Me = Ca. It is shown that the rhombohedral distortion of the perovskite type Lal_xMe• decreases while x increases. Lal_xMexCoO3_~ (Me = Sr, Ba) shows an ideal cubic structure at x -0.5. The stability range of (La1_yMey)2CoO 4 was found to be 0.25 < y < 0.35 for Me = Ca, 0.3 _ log(Po2) > -2.25 can be written as follows: Lal_ x, Sr• ~, = n Lal_x,,Srx,,CoO3_5,, + m SrCoO2.5 + q/2 O: where x' > x". The decomposition mechanism of Lal_xSrx CoO3_~ for the samples with x < 0.5 within the oxygen pressure range -2.25 > log(Po2) > -3.55 changes and can be written as follows: Lal_xSr• ~, = r Lax_x,Srx,COO3_5,, + w (Lal_y,Sry,)2CoO 4 + v CoO + f/2 02. The results are shown in "logPo2-composition" diagrams.

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