Abstract: The invention refers to a method for the non-invasive monitoring of the chemical reaction linked to the curing process of a thermoset plastic material using microwaves, from the real-time measurements of the reflection coefficient of a resonator sensor using a detector that simultaneously provides modulus and phase values. The invention also refers to, as an example of the application of the method, a device for the non-invasive determination of the degree of cure of a thermoset plastic material at microwave frequencies. In another aspect, the invention proposes a method for the determination of the unloaded resonance frequency and the unloaded quality factor of a very strongly coupled microwave resonator and a method for determining the complex permittivity of a material undergoing changes in density, such as those produced during the curing process of polyurethane.