The high-pressure solid solution and compression behavior of hydroxides in the ternary MgSiO H -AlOOH-FeOOH system were studied at pressures of 16–60 GPa and temperatures of 300–1513 K. We applied in situ X-ray measurements in conjunction with a multianvil apparatus using sintered diamond anvils to achieve homogeneous temperature distributions within large sample volumes. Our results show that CaCl -type hydroxides form solid solutions over a wide composition range. We also observed the volume reductions in the solid solutions accompanied by a spin transition of iron at ~50 GPa, and the wide compositional ranges are maintained through this process. Consequently, water may be transported into the deep lower mantle by the hydrous CaCl -type solid solution with changing its composition depending on the chemical and thermal environments. 4 2 2 2