Highly heat distortion resistant moulding compositions are disclosed comprising an aromatic polycarbonate having molecular weights Mw (weight average) of a least 10,000, containing bifunctional carbonate structural units corresponding to formula (Ia) ##STR1## wherein R.sup.1 and R.sup.2 denote, independently of one another, hydrogen, halogen, C.sub.1 -C.sub.8 -alkyl, C.sub.5 -C.sub.6 -cycloalkyl, C.sub.6 -C.sub.10 -aryl or C.sub.7 -C.sub.12 -aralkyl,m stands for an integer from 4 to 7,R.sup.3 and R.sup.4, which are selected individually for each X, denote, independently of one another, hydrogen or C.sub.1 -C.sub.6 -alkyl, andX denotes carbonunder the condition that R.sup.3 and R.sup.4 are both alkyl on at least one X atom andas the only thermo stabilizer, from 0.001 to 1% by weight of a siloxane corresponding to formula (II) ##STR2## wherein R and R' denote, independently of one another, C.sub.1 -C.sub.4 -alkyl or -aryl andx and y stand for the numbers 1,2 or 3 and x+y always add up to 4.