Binder phase (primarily chloride or nitrate) formation was examined in YX3-NaOH-H2O, Y2O3-acid-H2O, and Y2O3-salt-H2-O systems. The cementitious phase consisted mostly of plate- (or needle-) shaped hydroxy salts of the general formula Y2(OH)6-m XmnH2O, where m and n normally equal one. These binders were examined by X-ray diffraction and thermal analysis techniques. Nitrate binders decompose to Y2O3 by 600°C, whereas chloride binders form oxychlorides that sublime or convert to Y2O3 after oxygen replacement of chlorine (in air) at >1000°C. Although nitric and hydrochloric acid solutions form porous (< 50% dense) plasterlike bodies in 2 to 10 min of reaction with Y2O3 powder, salt solutions (i.e. NH4NO3 Mg(NO3)2, NH4Cl, and YCl3- ∼6H2O) slow the reaction considerably (48 h to 4 weeks), allowing 70- to 80%-dense cements to form. The effects of formation conditions on physical properties of binders were studied. Examination of scandium and lanthanide oxides showed that several behave in the same way as yttria.