The hafnium‐hydrogen system at room temperature consists of three phases. The lattice parameters of the hafnium metal used in this investigation and of the three phases are as follows: hafnium,a0=3.200A,c0=5.061A, andc=1.58; deformed cubic phase,a0=4.702±0.012A,c0=4.678±0.012A, andc=0.995; face‐centered cubic phase,a0=4.708±0.002A; and face‐centered tetragonal phase,a0=4.882±0.002A,c0=4.384±0.002A, andc=0.898. The deformed cubic phase extends up to HfH1.53and converts into the face‐centered cubic phase between HfH1.53and HfH1.70. The face‐centered cubic phase transforms into the tetragonal phase between HfH1.80and HfH1.87. The tetragonal phase extends up to HfH1.98.