The oxygen permeability of dense garnet-type Gd(3-x)A(x)Fe(5)O(12+/-delta) (A = Ca, Pr; x = 0-0.8) and Y3-x-yCaxNdyFe5-zNizO12-delta (x = 0-0.5; y = 0-0.25; z = 0-1.0) membranes at 1173-1273 K is limited by the bulk ambipolar conductivity. The ion transference numbers, calculated from results on the oxygen permeation and total conductivity, vary from 1 x 10(-5) to 5 x 10(-3), increasing with temperature. Ionic conduction in ferrite garnets, primarily determined by the oxygen vacancy concentration, increases with acceptor dopant additions. The activation energies for the oxygen ionic and electronic transport in air are in the ranges 176-224 and 20-81 kJ/mol, respectively. The ceramic microstructure of garnet-based materials has no essential effect on their ionic conductivity, which is low compared to that of perovskite-related ferrites. The low mobility of oxygen ions, probably limited by the ion transfer along the edges of Fe-O tetrahedra in the garnet lattice, is likely to result from a crooked diffusion pathway. Decreasing the A-site cation radius leads to a higher ionic conductivity of garnet phases. The thermal expansion coefficients of ferrite garnet ceramics at 300-1200 K in air are in the range (9.4-10.9) x 10(-6) K-1. (C) 2003 The Electrochemical Society.