This study investigates the activity and stability of molybdenum disulfide (MoS₂) as a hydrogen evolution reaction (HER) catalyst in proton exchange membrane water electrolyzers (PEMWEs). MoS₂ is considered a promising alternative to platinum, but its stability under HER conditions has been poorly understood. The research uses a scanning flow cell coupled with inductively coupled plasma mass spectrometry (SEC-ICP-MS) and electrochemical mass spectrometry (EC-MS) to monitor Mo and S dissolution. The study finds that the stability of MoS₂ is allotrope-dependent: lamellar-like MoS₂ is unstable under open circuit conditions, while cluster-like amorphous MoSₓ is unstable due to severe S loss during HER and the formation of undercoordinated Mo sites. The results provide guidelines for operating non-noble PEMWEs and propose an HER mechanism that accounts for Mo and S dissolution pathways. The study also highlights the importance of considering both activity and stability when evaluating the long-term durability of electrocatalysts.This study investigates the activity and stability of molybdenum disulfide (MoS₂) as a hydrogen evolution reaction (HER) catalyst in proton exchange membrane water electrolyzers (PEMWEs). MoS₂ is considered a promising alternative to platinum, but its stability under HER conditions has been poorly understood. The research uses a scanning flow cell coupled with inductively coupled plasma mass spectrometry (SEC-ICP-MS) and electrochemical mass spectrometry (EC-MS) to monitor Mo and S dissolution. The study finds that the stability of MoS₂ is allotrope-dependent: lamellar-like MoS₂ is unstable under open circuit conditions, while cluster-like amorphous MoSₓ is unstable due to severe S loss during HER and the formation of undercoordinated Mo sites. The results provide guidelines for operating non-noble PEMWEs and propose an HER mechanism that accounts for Mo and S dissolution pathways. The study also highlights the importance of considering both activity and stability when evaluating the long-term durability of electrocatalysts.