Abstract: The invention relates to carrying out thermal cracking of hydrocarbons, or other thermal conversions of organic substances in a reactor, for which a suitable reaction time is extremely short, e.g. of the order of milliseconds. Particulate solids are used as heat carrier and as feed an organic substance is used in the form of a gas which may contain some liquid; the hot particulate solids are introduced at low or no velocity into contact with the gas, which is at substantially higher velocity; the solids accelerate in passing through the reactor but the reaction is terminated substantially before the solids attain the velocity of the product gas. Contact times are short so that the solids do not accelerate to erosive speeds. The velocity differential enhances the heat transfer rate which makes short reaction times feasible.