A new compound of (Rb,K)(2)Cu-3(P2O7)(2) is obtained by high-temperature reactions from a mixture of RbNO3, KNO3, Cu(NO3)(2), and (NH4)(4)P2O7. The crystal structure was solved by direct methods and refined to R (1) = 0.056 for 5022 independent reflections. The compound belongs to a rhombic crystal system, P2(1)2(1)2(1), Z = 8, a = 9.9410(7) , b = 13.4754(6) , c = 18.6353 (3) , and R = 0.056. The basis of the structure is a complex copper-phosphate skeleton of the composition of [Cu-3(P2O7)(2)](2-), which can be regarded as consisting of two types of heteropolyhedral layers parallel to the (001) plane. The layers are alternated with each other, forming a frame, in the cavities of which the positions of alkali cations are located, statistically populated with K+ and Rb+ ions. Based on the refined populations of the positions of alkali cations, an exact chemical formula of the compound can be written as Rb1.28K0.72Cu3(P2O7)(2). The compound is the most complex among those known to this day of the composition of A(2) B-I(3) (II)(P2O7)(2) (A = Li, Na, K, Rb, or Cs; B = Ni, Cu, or Zn).