This paper ]presents a study of t h e transition to fast streamers in mineral oil i n overvolted point-plane gaps. Streamer propagation is studied in 5 cm and 10 cm gaps, either in t h e liquid alone or in the presence of solid surfaces parallel or perpendicular to the gap axis. The experiments presented concern the visualization o f streamers, the measurement of breakdowlti and acceleration voltages. In the liquid alone, a b o v e some critical voltage, a
large increas,e i n propagation velocity is recorded from 2 km/s up t o about 100 kmls. The presence of insulating s o l i d s parallel to the electric field is greatly in favor o f the inception and propagation of fast streamers (velocities up to 300 km/s have been measured).Conversely it is observed that a solid perpendicular to the electric field stops more efficiently f a s t streamers than slow ones, which easely creep rind get round such obstacles.