The reaction of [gamma-SiW10O36](8-) with Mn2+ and Ln(3+) in an aqueous solution led to the isolation of a series of new lanthanide-containing sandwich-type polyoxometalates (POMs) [{Ln(H2O)(n)}(2){Mn-4(B-alpha-SiW9O34)(2)(H2O)(2)}](6-) (1-5a) (Ln = La (1), Nd (2), Gd (3), Dy (4), Er (5); n = 5, 6), which crystallize in the space groups C2/c with a = 33.0900(2)-32.9838(15) angstrom, b = 12.8044(10)-12.7526(6) angstrom, c = 22.8273(17)-22.6368(11) angstrom, V = 9669.2(12)-9519.7(8) angstrom(3), Z = 2 (1, 2); P (1) over bar with a = 11.9502(4)-11.8447(6) angstrom, b = 13.2203(4)-13.1164(5) angstrom, c = 15.8291(5)-15.8524(7) angstrom, V = 2221.25(13)-2189.95(18) angstrom(3), Z = 1 (3, 4, 5), respectively. X-ray diffraction analysis reveals that they consist of two-dimensional networks based on a sandwich-type polyanion [Mn-4(B-alpha-SiW9O34)(2)(H2O)(2)](12-) (6a, {Mn-4(SiW9)(2)}) and lanthanide cations (Ln(3+)), which are further connected into three-dimensional frameworks by potassium cations for 3, 4, and 5. The unprecedented combination of time-resolved electrospray ionization mass spectrometry (ESI-MS) studies and X-ray crystallography allows us not only to directly observe the in-solution rearrangement of divant anion [gamma-SiW10O36](8-) into the sandwich-type POM 6a via an intermediate species [Mn-3(B-beta-SiW8O30(OH))(B-beta-SiW9O33(OH))(H2O)](12-) (7a, {Mn-3(SiW8)-(SiW9)}) from ESI-MS results, but also to gain the solid-state structures of intermediate and final product isolated from reaction solutions from X-ray crystallography results, from which the self-assembly mechanism of the lanthanide-containing sandwich-type POMs 1-5a was proposed.