A series of Sr-ion deficient perovskites La0.8Sr0.2-xGa0.8Mg0.2O2.8-delta (LSGM8282, x = 0.00, 0.05, 0.10, 0.15, 0.20), was synthesized by a conventional solid-state reaction method and their electric conductivity and chemical reactivity with Gd-doped ceria were investigated. Reactivity tests between the LSGMs and Ce0.9Gd0.1O2-delta (GDC) were carried out by X-ray diffraction, SEM-EDS, and electric conductivity measurements. The Sr-ion deficient LSGMs have a lower reactivity against the formation of high-resistivity phases than the stoichiometric (x = 0.00) LSGM. The reaction layer formed at the interface of LSGM and GDC during the sintering process due to the mutual diffusion of the cations was classified into five layers depending on the composition. The introduction of the Sr-ion deficient LSGM suppressed the formation of the highly resistive Sr-rich (La1+xSr1-x)Ga3O7-delta phase. It was suggested that the Sr-ion deficient LSGM (La0.8Sr0.2-xGa0.8Mg0.2O2.8-delta) of x = 0.15 was the best composition for suppressing the reaction with the GDC interlayer while retaining a relatively good electric conductivity.