Thecompound [Pr{(C6H11)2PS2}3] is triclinic, spacegroupP, with cell dimensions: a= 10.416(5), b= 12.327(4), c= 18.275(8)Å, α= 73.20(3), β= 79.50(4), and γ= 85.14(3)°, Z= 2. The analogous samarium compound [Sm{(C6H11)2PS2}3] is also triclinic, spacegroup P with cell dimensions: a= 9.664(3), b= 11.499(13), c= 20.612 (19)Å, α= 85.87(8), β= 77.59(6), γ= 84.61(7)°, Z= 2. The structures were determined from X-ray diffraction data collected with an automatic diffractometer and the model refined to R 0.052 (Pr, 3 617 reflections) and 0.079 (Sm, 2 467 reflections). Both Pr and Sm are co-ordinated to six sulphur atoms at mean distances of 2.84 and 2.79 Å. The co-ordination polyhedra are distorted trigonal prisms with different distortions. The Pr compound is isomorphous with the Nd analogue, and the Sm complex with those formed by Eu–Lu and Y.