In the last decade alternative insulating gases for gas-insulated systems (GIS) have been established on the market to lower the global warming potential of the electrical energy network. Partial discharge (PD) measurements are a typical diagnosis instrument to identify typical defects in GIS. One specific PD measurement method is measuring the frequency spectrum of the PDs in the ultra-high frequency (UHF) range using a spectrum analyser. This contribution presents UHF spectra measured in a fluoronitrile mixture as well as in carbon dioxide. Two typical defects were investigated: fixed protrusion on ground potential and free-moving particle. The results indicate that defect recognition is possible evaluating exclusively the UHF spectrum. A comparison with literature sources suggests similar UHF spectra for the analysed alternative insulating gases and sulphur hexafluoride when looking at the same defects.