Single crystals of R2Mo5O18 and R6MO12O45 (R = Eu and Gd), which are novel compounds in the R2O3-MoO3 system, have been obtained by thermal decomposition of [R-2(H2O)(12)Mo8O27].nH(2)O in air at 750 degreesC for 2 h. TG-DTA and X-ray diffractometry showed that R2Mo5O18 crystallizes in a melt of the dehydrated precursor (R2Mo8O27), and R2Mo5O18 is transformed to R6Mo12O45 in the solid state, both occurring with the loss of MoO3. R2Mo5O18 species crystallize isostructurallyas orthorhombic, Pbcn, Z = 4, with lattice constants of a = 19.2612(7) and 19.246-(1) Angstrom, b = 9.4618(3) and 9.4414(5) Angstrom, c = 9.3779(3) and 9.3446(4) Angstrom for R = Eu and Gd, respectively. R6Mo12O45 crystallize isostructurally as triclinic P (1) over bar, Z = 1, with lattice constants of a = 9.3867(4) and 9.3409(3) Angstrom, b = 10.9408(5) and 10.8826(5) Angstrom, c = 11.4817(5) and 11.4377(5) Angstrom, alpha = 104.194(2)degrees and 104.170(1)degrees, beta = 109.567(3)degrees and 109.288(4)degrees, gamma = 108.998(2)degrees and 109.266(2)degrees for R = Eu and Gd, respectively. Both structures consist of {RO8} square-antiprisms and {MoOn} polyhedra. In R2Mo5O18, an {RO8} polyhedron is attached by only molybdate groups, being isolated from adjacent {RO8} groups. The 12 nearest R atoms surrounding an R atom with (RR)-R-... distances of 6.0735(4)-7.0389(4) Angstrom form an approximate cuboctahedron. All the {RO8} square-antiprisms in R6Mo12O45 are connected to each other by face-sharing to form dimeric {R2O13} and {R2O12} groups. The latter unusual {R2O12} group is achieved by sharing a square-face via four bridging 0 atoms with a very short (RR)-R-... separation (3.4741(7) and 3.4502(6) Angstrom for R = Eu and Gd, respectively).