We have measured the volumes of dhcp- and fcc-FeHx at temperatures of 300–950 K and hydrogen pressures of 4–7 GPa using in situ X-ray diffraction. For dhcp-FeHx, slight reduction in volume starting at around 500 K on heating was observed in the V–T relations. This is likely related to magnetic transition. For fcc-FeHx, the expanded volumes per Fe atom with respect to fcc-Fe metal were converted to hydrogen compositions x using hydrogen-induced volume pansion vH. A miscibility gap in the x–T diagram of fcc-FeHx was confirmed to be in agreement with an earlier study, and the critical pressure was located in a rather narrow span of 4.0–4.5 GPa with a critical composition of x = ~0.4. We xamined Arrhenius plot analysis of the x–T relations to derive the enthalpy of solution of hydrogen delta-Hs. However, an irregular behavior of the x–T relations did not allow derivation of a reliable value of delta-Hs.