The novel quaternary reduced molybdenum oxides CaxR17-xMo19O46 (4 <= x <= 7; R = Ce, Pr, Sm, and Gd) have been synthesized by a solid-state reaction at 1400 degrees C for 48 h in sealed molybdenum crucibles. The crystal structure was determined on a single crystal of Ca5.24R11.76Mo19O46 by X-ray diffraction. This compound crystallizes in the monoclinic space group C2/m: a=19.5192(4)angstrom, b=11.1244(3)angstrom, c=13.2589(5)angstrom, beta=132.055(1)degrees, V=2137.7(1) angstrom(3), Z=2. Refinements yield R1 (F-2) = 0.0388 and wR2(F-2) = 0.0792 for 5667 unique reflections. The structure is built up from alternating slabs made up of Mo forming centered Mo-19 nu(2)-octahedral clusters, Ca, Pr, and O atoms and slabs containing only Ca, Pr, and O atoms. The Mo-19 cluster, in which the Mo-Mo distances range from 2.7274(4) to 2.7940(7) angstrom, results from a three-dimensional condensation of six Mo-6 octahedra. The Ca2+ and Pr3+ cations occupy seven crystallographically independent sites with coordination numbers in the O atoms varying from 6 to 8. Magnetic susceptibility measurements made on the CaxPr17-xMo19O46 (x = 4-7) compounds confirm the presence of Pr3+ cations, and no magnetic ordering was observed down to 4.2 K. Electrical resistivity measurements on a single crystal of Ca similar to 5Pr similar to 12Mo19O46 show a semiconducting behavior.