Perovskite-type oxides La(1-a)A(a)M(1-b)B(b)O(3-x) with A = Sr2+, Ln(3+), Ce4+, M = Fe, Co, Ga and B = Co, Fe, Mg were prepared in the concentration range a = 0.1-1 mol and b = 0.1-0.5 mol. Additionally, A-substoichiometric compositions were prepared. Preparation conditions for monophase materials and structure types of the perovskite were determined by X-ray investigation. The electrical conductivity as a function of p(O2) in the range 10(5) >p(O2) > 10(-14) Pa and temperature (500-1000 degrees C) was measured on ceramic shapes by a de four-point technique in combination with solid electrolyte coulometry. The ionic part of the conductivity in mixed conductors was determined by oxygen permeation measurements. The II-III perovskites Sr(Co,Fe)O3-x in their stabilized form are excellent mixed conductors (maximum 500 S cm(-1) at 400 degrees C) and have an up to two orders of magnitude higher oxygen ionic conductivity than the preferred III-III perovskite La(Sr)Mn(Co)O3-x. The oxygen ionic conductivity of the electrolyte La(Sr)Ga(Mg)O3-x could be increased by doping with 0.1 mol Co. By application of higher Co or Fe doping concentrations the lanthanum gallate becomes a mixed conductor.