We describe a thermometry system for low temperature calorimetric particle detectors, consisting of a phase transition thermometer combined with a SQUID readout. Studies were performed of a cadmium phase transition thermometer attached to a 1.1 g silicon absorber. Operated at 455 mK an energy resolution of ΔEE = 1% for irradiation with 5.8 MeV α-particles was achieved. This resolution was limited by the data acquisition system used. We also studied an iridium thermometer attached to a 280 g sapphire absorber operated at 135 mK. This calorimeter exhibited an energy resolution of ΔEE = 1.2% for irradiation with 5.8 MeV α-particles. The change in temperature caused by the absorption of one α-particles was measured to be 4.5 μK.