In high-pressure nanosecond surface dielectric barrier discharges (nSDBD) it is common to observe two different type of streamer development: a diffuse mode and a filamentary mode [1-2]. The transition between these modes was defined as the apparition of 2 to 5 filaments with varying applied voltage. In this study, the effects of photoionisation and electronegativity of the gas mixture on the transition voltage were compared for a wide range of pressures and concentrations. In the positive polarity, it was found that photoionisation played a stabilising role on the diffusive discharge for all studied pressures, while electronegativity appeared to reduce the threshold for transition at lower pressures. In the negative polarity, the photoionisation effect was no longer observable while the filament threshold was lowered for more electronegative mixtures. Fundemental understanding of these processes is of great importance in many applications, most of all in plasma-assisted combustion [3-4].