Single crystals of a recently prepared, at room temperature non-luminescent, linear-chain cl-uranyl phenylphosphonate (alpha-UPP), [UO2(HO3PC6H5)(2)(H2O)](2) . 8H(2)O, transform at room temperature and moderate humidity into a new strongly luminescent linear-chain beta-uranyl phenylphosphonate (beta-UPP), UO2(HO3PC6H5)(2)(H2O). 2H(2)O. Both compounds have also been synthesized separately and characterized by single-crystal methods. The alpha-phase crystallizes in the triclinic space group P (1) over bar, with a = 11.724(4) Angstrom, b = 16.676 Angstrom, c = 11.375 Angstrom, alpha = 101.61(5)degrees, beta = 106.76(3)degrees, gamma = 102.57(4)degrees, and Z = 2, whereas the beta-phase crystallizes in the monoclinic space group P2(1)/c with a = 7.766(2) Angstrom, b = 11.255(2) Angstrom, c = 20.959(4) Angstrom, beta = 100.38(3)degrees, and Z = 4. In both compounds, the coordination of the uranium atoms is pentagonal bipyramidal, and the metal-to-phosphonate ratio is 1:2. The most obvious difference between the two linear-chain compounds is the manner in which their phenyl rings are arranged. When viewed along the appropriate axes, phenyl rings of alpha-UPP arrange in two rows in a cis conformation, whereas those of beta-UPP arrange in two rows in a trans conformation. The suggested mechanism of transformation consists of breaking a U-O-equatorial bond, followed by a rotation of a phenylphosphonate group. During this rotation, the protonated phosphonate oxygen is deprotonated by one of the lattice water molecules and the detaching phosphonate oxygen is protonated. The deprotonation appears to be the rate-determining step of the transformation, not the U-O-equatorial bond breaking. The linear uranyl phenylphosphonate chains remain uninterrupted during the phase transformation process, but they shift and rearrange to form a different lattice as their shape changes from cis to trans.