High pressure photon echo results are presented for pentacene doped naphthalene, pentacene doped (triclinic and monoclinic) p-terphenyl, and rhodamine doped PMMA. Figure 1 illustrates typical photon echo intensity decays in a high pressure diamond anvil cell at variable low temperatures. The effect of pressure and temperature on the homogeneous dephasing is used to characterize the interactions between the impurity molecule and the host crystal, as well as provide a unique probe of local polymorphic phase changes.