[en] Highlights: • Comprehensive structural study on metastable Tb- and Dy-monazites. • First published Raman spectra of monoclinic TbPO₄ and DyPO₄. • Data fit well into linear trends of stable lanthanide phosphates. -- Abstract: Metastable Ln-orthophosphates (Ln = Tb, Dy) with monoclinic monazite-type structure were obtained via a precipitation route in aqueous solutions and subsequent thermal treatment. The lattice parameters and fractional coordinates of the atoms in the crystal structures were determined by Rietveld refinement of X-ray diffraction data. The lattice parameters of the metastable monazites obtained here fit well into the linear trends observed for stable LnPO₄ monazites (Ln = La–Gd). Infrared spectroscopic investigations on TbPO₄ and DyPO₄ with monazite structure confirmed the expected shift of the modes to higher wavenumbers with increasing atomic number and decreasing cation radius. Raman spectra of the metastable Tb- and Dy-monazites were obtained for the first time, revealing a typical monazite spectrum for monoclinic TbPO₄, continuing the general linear correlation between the Raman shifts and Ln³⁺ radii observed for the lighter LnPO₄ monazites. In contrast, the Raman spectrum of DyPO₄ was severely biased by fluorescence effects.