Compounds with the nominal formula K 2 [LnTa 3 O 10 ] (Ln = La, Nd, Pr), crystallising in a cation-deficient Ruddlesden–Popper structure, were prepared by the Pechini (polymeric complex) method. The crystal structures of the hydrated form of these compounds were determined by Rietveld analysis of the X-ray powder diffraction data and high-resolution transmission electron microscopy. The samples were also analysed to determine their specific areas (Brunauer–Emmett–Teller method) and degree of hydration (thermogravimetric analysis) and were tested as photocatalysts in the photocatalytic production of H 2 from water. The activity to produce H 2 from pure water decreased in the order Pr > La > Nd. The NiO x impregnation on the surface of the photocatalysts had no notable effect on the photocatalytic activity.