A new method, called the microwave-induced solution-polymerization synthesis (denoted as MW), and the conventional Pechini (denoted as PH) method have been used to prepare powders of La0.8Sr0.2Ga0.83Mg0.17O2.815 (denoted as LS0.2GM0.17) and La0.8Sr0.2(Ga0.9Co0.1)0.83Mg0.17O2.815(denoted as LS0.2GCM0.17). A higher heating rate and a more homogenous heating manner without thermal gradients in the microwave oven resulted in a pure LSGCM powder after calcination at 1400 °C for 9 hours (h) and purer LSGM (6.2% secondary phases). The grain size of the pellets of the above two powders was 2–3 μm without segregation. The densities of LS0.2GM0.17/MW and LS0.2GCM0.17/MW pellets, sintered at 1400 °C for 9 h, were 6.45 and 6.30 g cm−3, respectively.