This study investigates the selectivity of tungsten trioxide (WO₃) in photocatalytic methane (CH₄) oxidation to formaldehyde (HCHO). WO₃ is recognized as a promising photocatalyst for this reaction, but the underlying mechanisms remain unclear. The research focuses on two WO₃ facets, {001} and {110}, which exhibit distinct catalytic behaviors. In situ diffuse reflectance infrared Fourier transform spectroscopy, electron paramagnetic resonance, and theoretical calculations reveal that the {001} facet achieves 100% selectivity to HCHO through an active site mechanism, while the {110} facet generates HCHO along with intermediates via a radical mechanism. The {001} facet's high oxidation potential and low surface energy enable direct CH₄ oxidation to HCHO without intermediates, ensuring high selectivity. In contrast, the {110} facet preferentially activates H₂O and forms oxygen vacancies, leading to a lower HCHO selectivity. This study provides insights into the competitive catalytic pathways and guides the design of high-performance photocatalysts for selective CH₄ oxidation.This study investigates the selectivity of tungsten trioxide (WO₃) in photocatalytic methane (CH₄) oxidation to formaldehyde (HCHO). WO₃ is recognized as a promising photocatalyst for this reaction, but the underlying mechanisms remain unclear. The research focuses on two WO₃ facets, {001} and {110}, which exhibit distinct catalytic behaviors. In situ diffuse reflectance infrared Fourier transform spectroscopy, electron paramagnetic resonance, and theoretical calculations reveal that the {001} facet achieves 100% selectivity to HCHO through an active site mechanism, while the {110} facet generates HCHO along with intermediates via a radical mechanism. The {001} facet's high oxidation potential and low surface energy enable direct CH₄ oxidation to HCHO without intermediates, ensuring high selectivity. In contrast, the {110} facet preferentially activates H₂O and forms oxygen vacancies, leading to a lower HCHO selectivity. This study provides insights into the competitive catalytic pathways and guides the design of high-performance photocatalysts for selective CH₄ oxidation.