Luminescence signals have been recorded at low temperature during thermal cycling of crystals of RbTiOPO4 (RTP) and KTiOPO4 (KTP) excited by x-rays. In both crystals there are well resolved intense intensity spikes and changes in emission spectra consistent with first-order phase transitions. The transitions are not reversible, except by high temperature annealing. The observations imply that new phases of RTP and KTP can be formed, and in some cases these may relate to nanoparticle-size inclusions. Crystal quality assessment from the luminescence behaviour may have value for non-linear optics. Cathodoluminescence of the RTP surface reveals different emission spectra from the bulk material with consequent implications for usage in surface waveguide devices.