A series of compounds Ln2Ta 3Se 2O 8 ( Ln = La, Ce, Pr, and Nd) has been prepared and characterized. Pr 2Ta 3Se 2O 8 is orthorhombic, space group D122h - Pnnm, with unit cell constants a = 9.882(3), b = 11.774(3), c = 7.601(2) Å, T = 111 K, Z = 4. The structure has been solved and refined from single crystal X-ray diffraction data. The structure, including the oxygen content and positions, has been confirmed from neutron diffraction powder data. The asymmetric unit contains two independent Pr atoms, two independent Ta atoms, two independent Se atoms, and five independent O atoms. The Pr atoms are in tricapped-trigonal prismatic sites enclosed by seven O and two Se atoms. One Ta atom is in a distorted octahedral site surrounded by six O atoms, and the other Ta atom is also in a distorted octahedral environment but has two trans O and four Se atoms in its coordination sphere. The structure shows a three-dimensional arrangement and is built up from layers of the Pr tricapped-trigonal prisms alternating with layers of the Ta octahedra. The other members of the series, Ln = La, Ce, and Nd, have the same structure, as judged from the similarity of their X-ray powder patterns with that of Pr 2Ta 3Se 2O 8.