The Dynamic nuclear polarisation (DNP) enhanced nuclear magnetic resonance (NMR) results presented in this thesis demonstrate the functionality of a tunable and switchable gyrotron operating at 260 GHz with a 9 T spectrometer. This gyrotron was designed at the Swiss Plasma Center. Millimeter wave passive components were built by the startup company SwissTo12. Monochromatic as well as frequency swept cross effect DNP experiments validate the basic functioning and frequency tunability of the gyrotron. High power operation is tested on systems exhibiting the Overhauser and solid effects with the aid of a novel planar probe designed to mitigate dielectric heating. Microwave polarisation control using a Matching Optics Unit is verified using cross effect DNP. Continuous wave electron paramagnetic resonance (EPR) measurements performed on a home built quasi-optical spectrometer operating at 260 GHz on the same sample space as the NMR spectrometer are also presented. EPR spectra of hyperpolarising agents measured on this spectrometer are used to prepare for and subsequently analyse the DNP experiments. Preliminary studies on surface modified titanium dioxide nanopowders using EPR at 9 GHz, displaying significant g-factor shifts are reported.