[en] Solubility studies were conducted with HfO_2(cr) solid as a function HCl and ionic strength ranging from 2.0 to 0.004 mol kg"-"1. These studies involved (1) using two different amounts of the solid phase, (2) acid washing the bulk solid phase, (3) preheating the solid phase to 1400 C, and (4) heating amorphous HfO_2(am) suspensions to 90 C to ascertain whether the HfO_2(am) converts to HfO_2(cr) and to determine the solubility from the oversaturation direction. Based on the results of these treatments it is concluded that the HfO_2(cr) contains a small fraction of less crystalline, but not amorphous, material [HfO_2(lcr)] and this, rather than the HfO_2(cr), is the solubility-controlling phase in the range of experimental variables investigated in this study. The solubility data are interpreted using both the Pitzer and SIT models and they provide log_1_0 K"0 values of -(59.75±0.35) and -(59.48±0.41), respectively, for the solubility product of HfO_2(lcr)[HfO_2(lcr) + 2H_2O ↔ Hf"4"+ + 4OH"-]. The log_1_0 of the solubility product of HfO_2(cr) is estimated to be < -63. The observation of a small fraction of less crystalline higher solubility material is consistent with the general picture that mineral surfaces are often structurally and/or compositionally imperfect leading to a higher solubility than the bulk crystalline solid. This study stresses the urgent need, during interpretation of solubility data, of taking precautions to make certain that the observed solubility behavior for sparingly-soluble solids is assigned to the proper solid phase.