This paper describes progress towards the understanding of temperature effects in 1-3 piezoelectric composite transducers, carried out via a combination of experimental investigation and finite element (FE) analysis using the commercially available PZFlex code. The elastic properties and internal absorption of different passive materials are measured using a through transmission ultrasonic technique, with dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) being employed to evaluate the glass transition behaviour and specific heat capacities (Cp) respectively. The fillers are then incorporated into piezoelectric composite devices and the transducer performance measured over a wide temperature range by means of electrical impedance analysis and laser scanning of the active surface. The FE models are employed to predict the temperature distribution within such transducers as a function of constituent material properties and the data is correlated with the experimentally measured characteristics. The influence of glass transition temperature on viscoelastic properties is highlighted, along with the design compromises necessary to ensure effective high power performance.