This study investigates the activity-stability relationships of molybdenum sulfide (MoS₂) hydrogen evolution reaction (HER) electrocatalysts, focusing on their performance under HER conditions. MoS₂ is a promising alternative to platinum in proton exchange membrane water electrolysis (PEMWEs), but its stability under HER operation is not well understood. The study uses a scanning flow cell coupled with inductively coupled plasma mass spectrometry (SFC-ICP-MS) and electrochemical mass spectrometry (EC-MS) to monitor Mo and S dissolution during HER. The results show that MoS₂ stability is allotrope-dependent: lamellar-like MoS₂ is highly unstable under open circuit conditions, while cluster-like amorphous MoS₃₋ₓ is unstable due to S loss and undercoordinated Mo site generation. The study provides guidelines for operating non-noble PEMWEs based on stability metrics and proposes an HER mechanism that accounts for Mo and S dissolution pathways. The study also highlights the importance of monitoring both Mo and S species under HER conditions to understand the electrocatalytic processes responsible for HER active site generation. The results show that a-MoS₃₋ₓ presents the best trade-off between activity and stability for PEMWEs operating under constant load. The study also discusses the role of surface species under HER potentials in stability trends and the implications of MoSₓ catalyst structure on long-term HER activity and stability. The findings suggest that Mo-H hydride sites are likely responsible for HER activity in Mo-based electrocatalysts, and that the stability of these sites is highly dependent on the HER and dissolution pathways. The study concludes that MoSₓ-based PEMWEs would initially operate under low constant current loads when using c-MoS₂, while high-current operation even under intermittent mode would be suited for [Mo₃S₁₃]⁻-based catalysts. The study also highlights the importance of stability assessments in membrane electrode assembly (MEA) environments to fully estimate the effective lifetime of MoSₓ catalysts.This study investigates the activity-stability relationships of molybdenum sulfide (MoS₂) hydrogen evolution reaction (HER) electrocatalysts, focusing on their performance under HER conditions. MoS₂ is a promising alternative to platinum in proton exchange membrane water electrolysis (PEMWEs), but its stability under HER operation is not well understood. The study uses a scanning flow cell coupled with inductively coupled plasma mass spectrometry (SFC-ICP-MS) and electrochemical mass spectrometry (EC-MS) to monitor Mo and S dissolution during HER. The results show that MoS₂ stability is allotrope-dependent: lamellar-like MoS₂ is highly unstable under open circuit conditions, while cluster-like amorphous MoS₃₋ₓ is unstable due to S loss and undercoordinated Mo site generation. The study provides guidelines for operating non-noble PEMWEs based on stability metrics and proposes an HER mechanism that accounts for Mo and S dissolution pathways. The study also highlights the importance of monitoring both Mo and S species under HER conditions to understand the electrocatalytic processes responsible for HER active site generation. The results show that a-MoS₃₋ₓ presents the best trade-off between activity and stability for PEMWEs operating under constant load. The study also discusses the role of surface species under HER potentials in stability trends and the implications of MoSₓ catalyst structure on long-term HER activity and stability. The findings suggest that Mo-H hydride sites are likely responsible for HER activity in Mo-based electrocatalysts, and that the stability of these sites is highly dependent on the HER and dissolution pathways. The study concludes that MoSₓ-based PEMWEs would initially operate under low constant current loads when using c-MoS₂, while high-current operation even under intermittent mode would be suited for [Mo₃S₁₃]⁻-based catalysts. The study also highlights the importance of stability assessments in membrane electrode assembly (MEA) environments to fully estimate the effective lifetime of MoSₓ catalysts.