Although perovskites are extensively investigated in many areas, studies using perovskites as catalysts to activate oxidants for chemical oxidations are still quite limited. While various transition metals can be inserted into perovskites to form different perovskites, it is critical to investigate the effects of various transition metallic substituents on the activation of oxidants in chemical oxidation reactions. In this study, we propose to evaluate various metallic substituents in Lanthanum (La)-based perovskites (LaMO3 (M: Co, Cu, Fe and Ni)) for activating a strong oxidant, peroxymonosulfate (POMS), in order to degrade organic contaminants. Rhodamine B (RB) decolorization is used as a model test to evaluate generation of sulfate radicals from activation of POMS by LaMO3. LaCoO3 was found to exhibit the highest catalytic activity, followed by LaNiO3, LaCuO3 and then LaFeO3. LaCoO3 was then selected as a representative LaMO3 to be further investigated for the behavior of POMS activation under various conditions. LaCoO3-activated POMS was favorable under neutral conditions and at high temperatures, but less effective in the presence of NaCl. The mechanism of RB decolorization by LaCoO3-activated POMS was elucidated by examining the effects of radical inhibitors and attributed primarily to sulfate radicals and hydroxyl radicals to a lesser extent. We also found that both La3+ and Co3+/Co2+ ions contribute to catalytic decomposition of POMS for yielding sulfate radicals. LaCoO3 was also shown to activate POMS for RB decolorization over multiple trials without losing efficiency. These results reveal that LaCoO3 is a recyclable and effective La-based perovskite for POMS activation, which can be used for degradation of organic contaminants.