The invention relates to a process for preparing caprolactam by Beckmann rearrangement of cyclohexanone oxime by feeding cyclohexanone oxime to a reaction mixture comprising (i) sulfuric acid (ii) S03 and (iii) caprolactam, wherein the S03 content of the reaction mixture is between 9 and 20 wt. % and the molar ratio M of the reaction mixture defined as (nSO3+nH2SO4)/ncap is between 1 and 1.4, wherein nSO3=quantity of SO3 in reaction mixture, in mol nH2SO4=quantity of H2SO4 in reaction mixture, in mol ncap=quantity of caprolactam in reaction mixture, in mol.