A new series of heterometallic 3d-4f sandwich type phosphotungstates, [Ln{PCo2W10O38(H2O)(2)}(2)](11-) (Ln = Sm-III, E-III, Gd-III, Tb-III, Dy-III, Ho-III, Er-III, Tm-III, yb(III), and Lu-III, denoted 1a-10a, respectively), have been synthesized by a one pot reaction procedure on reacting the dilacunary K-14[P2W19O69-(H2O)].24H(2)O precursor with Ln(NO3)(3).nH(2)O and Co(NO3)(2). 6H(2)O in an aqueous potassium chloride solution. All the compounds were isolated as potassium salts and further characterized with different analytical techniques such as single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, high resolution electrospray ionization mass spectrometry, elemental analysis by inductively coupled plasma atomic emission spectroscopy, magnetic measurement, and thermogravimetric analysis. Single-crystal X-ray diffraction analysis of the compounds reveals that all these compounds are isostructural and crystallized in the orthorhombic crystal system in space group Iba2. The polyanions contain the alpha-1,5 isomer of dicobalt-substituted alpha-Keggin phosphotungstate, which sandwiched lanthanoid cation and formed novel heterometallic dimer species. The temperature dependent magnetic susceptibilities of la, 2a, 4a, and 7a-10a indicate the dominant contribution of the ferromagnetic interaction between Co-II and Co-II within the cluster, while the antiferromagnetic interaction between Co-II and Ln(III) dominates for 3a, 5a, and 6a. The isothermal magnetizations of 1a-10a show a gradual increase in magnetization at low fields and do not reach saturation even at 50 kOe.