Refractory high entropy alloys (HEAs) represent a new class of materials that show promising properties, such as high hardness, good thermal stability and sluggish diffusion. This makes them potential candidates for various applications, like for instance diffusion barriers in microelectronic devices. Within this study a series of refractory HEAs was deposited using high power impulse magnetron sputtering keeping the base alloy MoNbTaW constant and adding a fifth element, Ti, V, Cr, Mn, Zr or Hf, to investigate the influence of the alloying elements on the microstructure and properties of the films. The targets used for the synthesis of each alloy contained all five elements in an equimolar concentration to maximize the entropy within the HEAs. The chemical composition of the films roughly matches the composition of the targets. In general, the deposited films showed a body centered cubic (bcc) microstructure. The bcc microstructure of the Ti, V and Cr containing HEA films is stable up to 1200 °C. However, especially in the case of the MoNbTaWZr film, the deposition parameters could be tuned to enable the formation of amorphous films. An amorphous film is expected to have a better diffusion barrier performance, compared to a polycrystalline one, due to the missing grain boundaries, which can act as fast diffusion paths. To evaluate the diffusion barrier performance, 20 nm thick metal or nitride films were deposited on a Si substrate and covered by an additional Cu layer. The HEA nitride films could prevent barrier failure up to 800 °C, which is significantly better compared to the metal HEA diffusion barrier, where all barriers had already failed after annealing at 700 °C. From the results of the diffusion barrier investigations, design rules for metal and nitride HEA diffusion barriers can be derived: (i) for metals ¿The mixing enthalpy between Cu and all elements within the HEA diffusion barrier should be positive¿ and (ii) for nitrides ¿The nitride formation enthalpy of all elements within the HEA diffusion barrier should be as high as possible¿. Within this study, the great potential of these HEA and HEA nitride films for future diffusion barriers was demonstrated, in addition to the expansion of the knowledge of synthesis-structure-property relations of MoNbTaW-based films.