A process is described for preparing mixtures of diesters of phthalic acid with decanols and tridecanols by reacting phthalic acid or a reactive derivative of phthalic acid with a mixture made from at least one decanol and at least one tridecanol.Mixtures obtainable in this way are also described, as is their use as plasticizers for molding compositions, and also mixtures of isomeric tridecanols suitable for preparing the mixtures of diesters.