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By combining hydrogen and sulfur within diamond-anvil cells we synthesize (H2S)2H2 at 5 GPa and 373 K. Through a series of Raman spectroscopy, infrared spectroscopy, and synchrotron x-ray diffraction experiments we have constrained the phase diagram of (H2S)2H2 within a wide P−T range. On compression we observe the phase transition sequence of I-II-II′-III, where II′ is a previously unreported phase; at room temperature this sequence spans from 5 to 47 GPa, while the application of low temperatures stabilizes this sequence to 127 GPa (<80K). Above these pressures we propose that phase III of (H2S)2H2 transforms to a nonmolecular H3S network. Our Raman and infrared measurements indicate that the transition from (H2S)2H2 to H3S is reversible at room temperature. X-ray diffraction reveals that the symmetry of the underlying S lattice of (H2S)2H2 and H3S is retained along this compression path up to at least 135 GPa.