The search for Li ions conducting ceramics is burgeoning, owing to the regain interest for solid state batteries. Here we investigate the effect of Mg substitutions on the ionic conductivity of the A(2)SnO(3) (A = Li, Na) phases. Pure A(1.8)Mg(0.1)SnO(3) and A(2.2)Mg(0.1)Sn(0.9)O(3) were structurally characterized and their ionic conductivity was measured by AC impedance spectroscopy. We show a decrease of the activation energy with increasing the Mg substitution and found ionic conductivities three and two orders of magnitude higher for Li2.2Mg0.1Sn0.9O3 and Na1.8Mg0.1SnO3 as compared to pristine Li2SnO3 and Na2SnO3, respectively. Neutron diffraction was used to determine the Mg localization in the crystal structure and to provide a rationale for the ionic conductivity changes. Our results confirm the high sensitivity of the ionic conductivity on chemical substitutions, even limited ones.