1,2-Bis[bis(methoxyethyl)phosphino]ethane dioxide (tmepeO2) (1) and its molybdenum(VI) complex, MoO2Cl2 (tmepeO2) (2) are resistant to acidic hydrolysis in ambient temperatures. The crystal structure of 2 has been determined by X-ray diffraction methods and refined by a full-matrix least-squares technique to R = 0.0395 for 2393 independent non-zero reflections. Crystals are monoclinic, space group P2(1)/c, with four complex molecules in the unit cell of dimensions: a = 14.655(13), b = 8.298(10), c = 20.644(28) angstrom, beta = 108.97(9)-degrees.