In this work, the application of a new, synthesized niobium-based catalyst, called S4 (niobium oxyhydroxide), in the liquid-phase oxidation of methyl-phenyl sulfide (thioanisole) using hydrogen peroxide as oxidant was proposed. The synthetic method employed provided a material with low crystallinity and high specific surface area and acidity. A commercial material, called HY-340 (hydrated niobium oxide), was also employed as heterogeneous catalyst for comparative purposes. The results showed that the synthesized S4 material is an outstanding catalyst, being able to completely convert the substrate (thioanisole) that achieves almost 90% of selectivity for methyl phenyl sulfone formation, under mild reaction conditions. According to the theoretical and experimental combined results, the superior performance of S4 catalyst is related to the better interaction of H2O2 and thioanisole molecules with S4 surface, compared to HY-340, pointing to the greater ability of this catalyst to form reactive oxygen species in contact with hydrogen peroxide, due to its higher content of free hydroxyl groups present on its surface.