Irradiating a crystal of [N(C5H11)(4)](2)[Pt-2(H)(2)(pop)(4)], (pop = pyrophosphate), with a xenon lamp on a diffractometer, the intensity data were collected and the structure was analyzed by Xrays at low temperatures. The unit-cell volume and the Pt-Pt distance of the Pt anion became significantly smaller than the corresponding ones without irradiation. The Pt-Pt bond shortening is due to the formation of the excited molecules in the crystal, although the amount of the excited molecules may be about 5%.