K(3)Sb(3)A(2)O(14) . 5H(2)O (A=P, As) were synthesized by use of a hydrothermal method, and the synthesis conditions are discussed in detail. Both phases were characterized by various techniques including scanning electron microscopy, XRD, XPS, P-31 MAS NMR, IR, DTA-TG. The unit-cell parameters for K3Sb3P2O14 . 5H(2)O and K3Sb3As2O14 . 5H(2)O are slightly smaller than those for the corresponding phases synthesized by solid-state reactions.