A unified synthetic strategy has allowed us to rationalize the preparative chemistry of the layered oxovanadium phosphates M(VOPO(4))(2).nH(2)O. Thus, we have been able to isolate as single phases with reasonable yields both all the previously characterized phosphates and a new solid containing Ba(2+) cations as guest species as well as to prepare new related derivatives involving arsenate anions. In order to organize the experimental results, we have used two complementary models: a simple restatement of the partial charge model (PCM), and the valence matching principle (VMP) (derived from the bond valence method). The crystal structure of the new barium lamellar derivative, Ba(VOPO(4))(2).4H(2)O, has been solved from X-ray single crystal data. The cell is monoclinic (space group Pn; Z = 1) with a = 6.3860(3) Å, b = 12.7796(9) Å, c = 6.3870(5) Å, and beta = 90.172(6) degrees. Its structure, like it occurs with the other members of the M(VOPO(4))(2).nH(2)O family, can be thought of as derived from that of the well-known lamellar solid VOPO(4)(.)2H(2)O. Ba(2+) cations are located in the interlamellar space with an environment defined by 12 oxygen atoms. A comparative study of this family shows significant crystallochemical correlations with the radius of the guest cations.