Here, we report on ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) experiments aimed at exploring the complex surface interactions and dissolution behaviors of nanofillers in nanocomposites under high-salinity conditions pertinent to desalination and mineral recovery. In situ AP-XPS analysis at solid-liquid interfaces under near-ambient conditions provided experimental proof of salinity-induced partial dissolution and interactions with chloride ions, revealing the formation of complex surface-bound species. Transition-metal chalcogenides NiSe and CoSe were specifically selected as model nanofillers due to their potential in enhancing performance for membrane distillation (MD) and membrane crystallization (MCr) processes. Complementary density functional theory (DFT) simulations provided a detailed mechanistic understanding, offering a robust predictive framework validated by our …