The phthalic acid diesters and ester-amides according to the invention, and also the corresponding starting materials of the formula IIcan, however, also be employed in the form of mixtures of the 3- and 4-isomers.Preferred compounds of the formula I are those in which R.sub.1 is --CH.sub.2 CH.dbd.CH.sub.2, R.sub.2 is methyl and especially hydrogen and R.sub.3 and R.sub.4 are each alkyl having 1-8