In this paper, we discuss the use of broadband microwaves (MW) (up to 20 GHz) to characterize the thermal stability of dielectric thin films for emerging nanoelectronic applications. We will illustrate the talk with examples from our on-going work on the thermal stability of organic and hybrid silicon-organic thin films meant for insulation applications in micro- and nanoelectronic devices. We will illustrate our use of MW propagation characteristics to extract changes in electrical and mechanical properties, and their links to the changes in chemistry of prototypical materials. These studies shine light on the chemical changes that occur within the dielectric films that could impact the performance and reliability, as well as provide basis for rational selection of organic dielectrics for integrated devices.