The invention relates to a process for the preparation of acetylene and synthesis gas by thermal treatment of a starting mixture containing one or more hydrocarbons and in addition molecular oxygen and/or one or more compounds containing the element oxygen, in which the starting mixture is heated, brought to reaction in a reactor and subsequently cooled. The process has the special feature that the starting mixture is heated to a maximum of 1400.degree. C. It is then possible to carry out the process with comparatively little expenditure of energy.