Waste sunflower oil on rags left in a plastic bin created a chemical reaction that caused a shop with flats above in Upper Tooting Road, in Tooting.
The reaction, known as exothermic oxidation, was at the origin of the blaze that damaged part of the ground floor shop and the first floor flat.
The alarm was raised by neighbours and four people left the adjacent property as a precaution before the arrival of the Brigade.
The Brigade was called at 2320 and the fire was under control at 0214. Four fire engines and 21 firefighters and officers from Tooting, Wimbledon, and Clapham fire stations attended the scene.