Selected LnBa(0.8)Sr(0.5)Co(1.5)O(5 + delta) (Ln - Pr, Nd, Sm, Gd and Y) oxides were studied in terms of their phase composition, formation of cation-ordered crystal structure, oxygen nonstoichiometry and transport properties. Despite partial substitution of bigger Ba2+ by smaller Sr2+ cations, A-site (Ln-Ba,Sr) cation ordering in LnBa(0.5)Sr(0.5)Co(1.5)Fe(0.5)O(5 + delta) is preserved, but only for smaller Ln cations (Y3+, Gd3+). In the case of SmBa0.5Sr0.5Co1.5Fe0.5O5 + delta, the synthesis procedure yielded material partially ordered, while for Pr3+ and Nd3+ cations XRD measurements indicated formation of cation-disordered samples. Oxygen content in the disordered materials is close to stoichiometric one (delta approximate to 1) at room temperature, while for cation-ordered samples delta < 1. Thermogravimetric measurements indicated that above 250 degrees C a decrease of mass occurs for all of the studied materials. Except for Y-containing sample, larger mass loss at 800 degrees C occurs for materials with heavier Ln element. The ordered materials show higher activation energy of the electrical conductivity in 25-300 degrees C temperature range, as well as different temperature dependence of Seebeck coefficient, comparing to the disordered ones. The highest electrical conductivity was measured for PrBa0.5Sr0.5Co1.5Fe0.5O5 + delta. For this compound the ionic conductivity at 800 degrees C was determined to be of the order of 0.03 S cm(-1). (C) 2013 Elsevier B.V. All rights reserved.