The transition behaviours of PMMA-based gels with and without nano-particle filler have been investigated at elevated pressures up to 0.9GPa and in the temperature range 220–310K. Both gels had molar ratios of 53.9:22:5.2:18.9 of ethylene carbonate (EC), propylene carbonate (PC), lithium perchlorate, PMMA, and one gel contained 8wt.% TiO2 nano-particles. The results show a sluggish transition between a low-pressure (high temperature) state and a high-pressure (low temperature) state. The nano-particle filler facilitates the kinetics of the transition, yielding a significantly lower transition pressure as well as a better homogeneity in the sample after a transition back to the low-pressure state. These results can be explained by a filler-induced increase of the nucleation rate, which yields smaller crystallites. The transition is observed also by differential scanning calorimetry and Raman spectroscopy at ambient pressure and is likely associated with crystallization of the EC–PC solvent. The logarithmic pressure derivative of the conductance and, consequently, the activation volume changes a factor of 3 as a result of the transition.