Compounds of formula I or II ##STR1## in which n is 1 or 2, R.sup.1 is phenyl, substituted phenyl, alkyl, substituted alkyl, hydrogen, --OH or amino, R.sup.2 is hydrogen, R.sup.3 is hydrogen, alkyl, substituted alkyl, phenyl or substituted phenyl, R.sup.4 is alkyl, phenyl, substituted phenyl, alkylene or arylene, X is --O--, --S--, --CH.sub.2 -- and Y is a direct bond or --CH.sub.2 -- are latent hardening catalysts for acid curable resins. The catalysts are activated by heat and are used in heat-hardening high-solids industrial varnishes and for coil-coatings, can and wood varnishes.