The melting curve of NH 4HF 2 I rises from 125.2°C at atmospheric pressure to a triple point II/I/liquid at 9.3 kbar, 220°C. The I/II phase boundary is terminated at a triple point III/I/II at ∼45 kbar, 295°C. The melting curve of the new phase NH 4HF 2 II passes through a broad maximum at ∼39 kbar, 306°C, and is terminated at a triple point III/II/liquid at 46.3 kbar, 301°C. The melting curve of NH 4HF 2 III rises with pressure. The NH 4HF 2 III may be a dense hydrogen-bonded phase. Liquid NH 4HF 2 appears to be anomalous in several respects, and has a high compressibility relative to the solid phases.