GdCoO3 nanoparticles with sizes of 3, 12, and 200 nm were prepared from single source mixed-metal organic framework (MOF) precursor, [Gd(H2O)3Co{C5N1H 3C(COO)2}3]. The photocatalytic degradation studies, carried out for the first time in the presence of nanosized GdCoO 3 oxides, on dyes, phenols, and substituted phenols indicate favorable activity. The intermediates formed during the photocatalytic reactions were analyzed, and possible reaction mechanisms have been proposed. The results of the present study indicate that the degradation appear to be faster in the presence of 3 nm GdCoO3 compared to P-25 Degussa TiO2 catalyst. The reduced formation of unwanted intermediates in the presence of GdCoO3 indicates that this material would prove to be a good photocatalyst in environmental photocatalysis. © 2007 American Chemical Society.