The metamict fergusonite-(Y) with the formula (Y0.70Ln0.20Ca0.13U0.02Th0.02)∑1.07(Nb0.72Ta0.17W0.06Ti0.04)∑1(O3.97(OH)0.11F0.08Cl0.03) · 2.12H2O from the Blyumovskaya Pit, Ilmeny Mountains (Russia) was studied by the means of high-temperature X-ray diffraction, thermal analysis, Raman spectroscopy and microprobe analysis. Thermal expansion was studied for both tetragonal (α-fergusonite) and monoclinic (β-fergusonite) polymorphs. The expansion of β-fergusonite is anisotropic and strongly negative along the α33. In contrast, α-fergusonite exhibits a relatively isotropic thermal expansion upon heating. The volume CTE (αV) for β-fergusonite varies in the range 22.87(94)–75.4(2.5) × 10–6 ºC−1, whereas α-fergusonite has αV = 32.33(57)–31.66(49) × 10-6 ºC−1 in the temperature range 850–1200 °C. After heating to 1100 °C, the mineral develops a porous texture, and the radioactivity is reduced by 37%, which can be attributed to the partial volatilization of some radionuclides. In situ experiments revealed the complete sequence of the thermal evolution of the metamict fergusonite-(Y) for the first time.