Reinforced polyolefinic thermoplastic composition comprising a polyolefin, a carbonate of a metal appertaining to IIa Group of the Periodic System and a maleamic silane having general formula: ##STR1## wherein R and R.sub.1 are hydrogen or an alkyl radical; R.sub.2 is an alkoxy hydrolysable group or a halogen atom; R.sub.3 is an alkyl radical having from 1 to 8 carbon atoms; X may be a radical of formula: ##STR2## or an aromatic, cycloaliphatic o heterocyclic divalent radical.