Thermally induced optical opacity in macroscopic potassium dihydrogen phosphate (KDP) single crystals is a significant issue in the application of these materials in optical systems subjected to adverse environments. The present study investigates the relationship between thermally mediated structural transformations in KDP (associated with both a high-temperature phase transition and lattice dehydration effects) and the development of microcracking-induced optical scattering. In this case, the kinetics of these processes are examined under elevated-temperature, isothermal conditions. A significant temperature-dependence in the elapsed time required to observe both the structural transformation (via in situ Raman spectroscopy) and crystal opacification (via single-beam optical transmission) is observed. The phenomena are further correlated spatially within partially transformed specimens examined using optical microscopy and micro-Raman spectroscopy, demonstrating the inherent heterogeneous nature of the structural transformation in these bulk crystals. In this study, a change in the isothermal hold temperature of only 10 degrees C about the phase transition point decreased the elapsed time to transformation by nearly two orders of magnitude, significantly impacting an evaluation of optical performance in these materials under thermally variable conditions.