The invention relates to a method for the production of a mixture containing positionally isomeric aliphatic tertiary isononanoic acids based on 2-ethylhexanol by means of a reaction with concentrated formic acid and a concentrated Bronsted acid, preferably sulphuric acid, at a temperature of between 0 and 40 DEG C. Between 2 and 10 mol of formic acid and the same amount of concentrated Bronsted acid are added per mol of 2-Ethylhexanol, corresponding to between 6 and 90 mol of protons. The obtained reaction mixture is brought into contact with water and the formed mixture containing tertiary isononanoic acids is separated.