Abstract: The method of preparing sulphuric acid comprises distilling sulphur trioxide with subsequent sublimation condensation by cooling with a coolant having a temperature of from -5.degree. to -15.degree. C. The obtained sulphur trioxide is then treated with sulphuric acid having a concentration in the range of 95 to 96 per cent. The thus prepared sulphuric acid contains impurities in the range of 10.sup.-7 - 10.sup.-8 per cent by weight of cations, and 5.times.10.sup.-5 per cent by weight of anions, SO.sub.3 " in particular.