Dielectric material ageing is one of the main issues which induces electrical machine failure. This ageing is most generally induced by thermal and electrical stresses, oxidation, or a combination of these different effects. Ultra-High Frequency (300 MHz-3 GHz) characterization may be a promising method for the insulation ageing assessment, by monitoring the evolution of the dielectric properties in this frequency band. This characterization can be performed using electromagnetic sensors. Therefore, the main topic of this study focuses on the relative permittivity evolution of epoxy resin samples exposed to thermo-oxidative stress measured by a microstrip ring resonator. The first step is to determine if the sensitivity of this sensor is sufficient to identify a change in the dielectric properties of the resin samples. In order to correlate these potential changes induced by ageing phenomena, Fouriertransform infrared spectroscopy and mass loss measurements are also performed.