For the last couple of decades, catalytic converters (CC) have become a standard part of the internal combustion engine exhaust systems. Besides reducing toxic components in exhaust gases, catalytic converters can have a certain effect on the acoustic performance of the exhaust system. In this paper the sound transmission and attenuation in the catalytic converters has been investigated. A catalytic converter is known to have two distinct acoustic effects: the reactive effect originating from the acoustic wave reflections caused by cross-sectional area changes within the unit and the resistive effect which results in the acoustic wave dissipation caused by visco-thermal losses. The flow resistance in the narrow tubes in the catalytic converter element results in frequency dependent dissipative effects on the transmitted sound. An experimental investigation on engine catalytic converters treated as acoustic two-ports is carried out. The acoustic behavior of the catalytic converters is investigated in detail by analyzing the scattering matrix elements.