We report structural studies on Mo2P4O15 over the temperature range 16-731 K, which show that it is considerably more complex than revealed by earlier work, Its low-temperature structure has lattice parameters a = 24.1134(6) angstrom, b = 19.5324(5) angstrom, c = 25.0854(6) angstrom, beta = 100.015(1)degrees, and V = 11635.0(5) angstrom(3) at 120 K, containing 441 unique atoms in space group Pn, a remarkably high number for a material with such a simple composition. Mo2P4O15 undergoes a structural phase transition at similar to 520 K to a high-temperature phase in space group P (1) over bar, with lattice parameters a = 17.947(3) angstrom, b = 19.864(3) angstrom, c = 21.899(3) angstrom, alpha = 72.421(3)degrees, beta = 78.174(4)degrees, gamma = 68.315(4)degrees, and V = 6877.2(19) angstrom(3) at 573 K. The high-temperature structure, with 253 unique atoms, retains much of the low-temperature complexity.