The direct electrocatalytic epoxidation of propylene into propylene oxide (PO) is a promising sustainable approach in comparison to current industrial methods for propylene oxidation. However, the yield rates and selectivities of this reaction are too low for industrial requirements. Therefore, it is important to understand the reaction mechanisms for rational catalyst design. In this regard, two noble metal oxides, PtOx and PdOx, are considered in this work for investigating the electrooxidation of propylene into PO. PO is measured in situ via an electrochemical mass spectrometry instrument to overcome the ex situ detection challenges caused by the PO being unstable in aqueous media. In this way, PO could be measured before undergoing hydrolysis or other possible transformations. The selectivity toward PO reaches the maximum at 1.3 V vs RHE on PdOx, whereas on PtOx, the vertex occurs at 1.6 V vs RHE. Moreover, the activity trends of PO production indicate that on PdOx, the rate-determining step for the epoxidation is different whether one is more anodic or more cathodic than 1.3 V vs RHE, while it remains the same in the 1.0–1.6 V vs RHE potential window on PtOx.