Oxygen-ion conductivity of the perovskite-type solid solutions (La,Sr)Ga1-zM2O3-delta (M = Ti, Cr, Fe, Co; z = 0-0.20), LaGa(1-y-x)Mg(y)MiO(3-delta) (M = Cr, Fe, Co; y = 0.10-0.20, z = 0.35-0.60) and LaGa1-zNizO3-delta (z = 0.20-0.50) was studied using the techniques of oxygen permeation, Faradaic efficiency, ion-blocking electrode and the e.m.f. of oxygen concentration cells. Oxygen-ion transference numbers vary from 2 x 10(-6) to 0.98 throughout the series and p-type electronic conductivity increases with increasing transition metal content. Substitution of Ga with higher valence cations (Ti, Cr) decreases ionic conductivity whereas small amounts of Fe or Co (similar to 5%) increase ionic conductivity. For higher transition metal contents, lower levels of oxygen-ion conductivity and an increase in the activation energy, E-A, for ionic transport, from 60 (5%-doped) to 230 kJ/mol ( > 40%-doped) are observed. In heavily doped phases, E-A tends to decrease with temperature and, above 1170 K, values are similar to the undoped phase suggesting that an order-disorder transition takes place. Factors affecting the observed ionic conductivity trends are discussed.