The advantages of microwave heating in industrial processing are becoming more widely appreciated and the technique is of increasing commercial significance. However, knowledge of a material's dielectric properties as a function of temperature is of considerable importance as they determine the efficiency with which the material converts microwave energy into heat. This article describes the development of a novel instrument for a new form of thermal analysis, microwave dielectric thermal analysis (MDTA) in which the sample is heated not conventionally but by microwaves, while its dielectric properties at microwave frequencies are determined quasisimultaneously using a network analyzer. A plot of the dielectric properties against temperature then gives the MDTA curve. The fundamental principle underlying the use of MDTA is that not only do a material's dielectric properties (both the real and imaginary parts of the complex permittivity) alter (generally smoothly) as a function of temperature but dramatic differences can be found when chemical or physical changes occur. MDTA measures the sample's dielectric properties as a function of temperature before, during, and after the chemical and/or physical changes occurring as it is heated in the microwave field and thus provides a unique way of studying them. Furthermore, due to the quasisimultaneous nature of the measurements, MDTA can also make a valuable contribution to the investigation of the so-called "microwave effect" anomalies that occur when certain materials are heated in a microwave field. We show that the technique can be used to study a wide range of thermal transformations, including solid-solid phase changes, melting, and chemical reactions, e.g., dehydration, via the complex permittivity-temperature profiles generated in MDTA.