Two high-nuclearity lanthanide-transition metal clusters with the general formula [Ln(18)Co(II)Co(6)(III)(OH)(14)(CO3)(9)(CH3CH2COO)(6)(dea)(12)(H2O)(30)]center dot(NO3)(8) center dot Cl-4 center dot(CH3CH2OH)(6)center dot H2O)(12) (Ln(18)Co(7), Ln = Gd (1) and Dy (2)) have been obtained by reacting CoCl2 center dot 6H(2)O, Ln(NO3)(3)center dot 6H(2)O, and a mixture of ligands consisting of propionate and diethanolamine (H(2)dea). Crystal structural analysis exhibits two three-blade propellers composed of the Co-3(III) Ln(9) units connected by one Co-II ion and three CO32- ions, which assemble into a double-propeller-like structure (Ln(18)Co(II)Co(6)(III)). Magnetocaloric effect (MCE) studies indicate that Gd18Co7 exhibits a large entropy change (-Delta S-m) of 36.9 J kg(-1) K-1.