The condition of the formation of Y(III) and lanthanide (III) (La-Lu) isocinchomeronates were studied, and their quantitative compositions and solubilities in water at 293 K were determined. The IR spectra of the hydrated complexes were recorded and their thermal decomposition in static air were determined. During heating the hydrated isocinchomeronates Ln(2)(C7H3NO4)(3) . nH(2)O (n = 6-20) lose some (Ce, Pr, Dy-Tm) or all (Y, La, Nd-Tm, Dy, Lu) of their crystallization water molecules in one (Y, La, Pr-Dy, Er, Yb, Lu) or two (Ce, Ho, Tm) steps then the hydrated complexes decompose directly to the oxides (Y, Ce, Pr-Lu) or with intermediate formation of oxocarbonates Ln(2)O(2)CO(3) (La).