an exhaust gas purification system for internal combustion engines having at least two cylinders of which the port of each exhaust valve communicates with a port passage provided in the cylinder head , and a pair of the adjacent port passages communicate with a common port passage forming a siamese port passage therewith . a secondary air intake passage having a check valve communicates with the port passages for introducing secondary air by exhaust pulsation . in order to enhance the exhaust pulsation effect , the exhaust pipe connected to the opening of the common port passage has a constant cross section of a fixed diameter . heat insulation is provided along the siamese port passage and a portion of the exhaust pipe for good oxidation of the unburned constituents . the total volume of the exhaust passage through the siamese port passage and the heat insulated exhaust pipe portion is equal to the displacement volume of the corresponding cylinders for introduction of a sufficient amount of the secondary air . the ignition timing is retarded to restrict the combustion temperature to under 2000 ° c . for reducing the amount of nitrogen oxides .