The invention relates to a process for the regeneration of used sulfuric acid which is contaminated by secondary constituents from the group consisting of methylsulfuric acid, dimethyl ether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbons and organic silicon compounds, in which the used sulfuric acid is 1. diluted to a concentration of not more than 55% by weight of sulfuric acid by addition of liquid water and introduction of steam and heated to boiling at not more than 135 DEG C and 2. concentrated to at least 80% by weight under reduced pressure and at a temperature of at least 170 DEG C and then reacted with an oxidant.