Three rare-earth oxide selenates Ln₂O₂SeO₄ (Ln = La, Pr, Nd) have been prepared via double-exchange solid-state reactions between respective LnOCl oxyhalides and potassium selenate. This approach succeeded to obtain single-phase specimens of La₂O₂SeO₄ and Nd₂O₂SeO₄, previously known as transients upon thermal decomposition of the corresponding selenates, as well as a new compound Pr₂O₂SeO₄. Refinement of their crystal structures from powder X-ray diffraction data confirmed previous attributions to the grandreefite (Pb₂F₂SO₄) structure type observed also for the Ln₂O₂SO₄ oxide sulfates. According to polythermic X-ray studies, La₂O₂SeO₄ is stable until at least 700 °C. All compounds were characterized by infrared and X-ray photoelectron spectroscopy.