A process for the preparation of hydroxybenzylphosphonates of forula ##STR1## by reacting Mannich base of formula ##STR2## with as trialkyl phosphite of formula ##STR3## wherein R.sub.1 and R.sub.2 are each independently of the other C.sub.1 -C.sub.12 alkyl or, when taken together form a piperidinyl or morpholinyl radical, and R.sub.5, R.sub.6 and R.sub.7 are each independently of one another C.sub.1 -C.sub.4 alkyl, which reaction is carried out in the presence of a carboxylic anhydride (IV).The compounds of formula I may suitably be used as processing stabilizers for plastics.