The invention relates to Si-modified phthalic acid derivatives (acids, esters and anhydrides) of the formula I ##STR1## in which R.sub.1 and R.sub.2 independently of one another are --OH, alkoxy with 1-12 C atoms or phenoxy or R.sub.1 and R.sub.2 together are the --O-- group, X is a ##STR2## R.sub.3 is alkyl with 2-7 C atoms, cycloalkyl with 5-7 C atoms, benzyl or ##STR3## Q is methyl, phenyl or --OQ.sub.3 and Q.sub.1, Q.sub.2 and Q.sub.3 independently of one another are alkyl with 1-6 C atoms or phenyl. The products of the invention are prepared by reacting the corresponding allylamino-phthalic anhydrides or allylamino-phthalic acid esters with compounds of the formula III ##STR4## They are used as adhesion promoters, for example between inorganic solids and organic resins. They may also be used for the preparation of other Si-modified adhesion promoters, and as curing agents for epoxide resins.