The subject matter of the invention is a process for the preparation of carboxylic acids and derivatives of them of the general formula ##STR1## wherein R means hydrogen, or a C.sub.1-4 alkyl or a (C.sub.1-5 alkoxy)carbonyl group,R.sub.1 is as defined in claim 1,R.sub.7 stands for hydrogen or a C.sub.1-7 alkyl group andR.sub.8 means hydrogen or a carboxyl group,by reacting a 1,3-dioxane-4,6-dione derivative of the general formula ##STR2## wherein R.sub.9 stands for a C.sub.1-4 alkyl group or a phenyl group, optionally monosubstituted by halogen andR.sub.10 stands for hydrogen or a C.sub.1-5 alkyl group orR.sub.9 and R.sub.10 together form a pentamethylene group,and an aldehyde or ketone of the general formula ##STR3## in the presence of formic acid.