The ultrasound-induced cleavage of macromolecules has become a routine experiment in the emerging field of polymer mechanochemistry. To date, it has not been conclusively proven whether the molecular weight of a polymer or its contour length is the determining factor for chain scission upon ultrasonication. Here we report comparative experiments that confirm unequivocally that the contour length is the decisive parameter. We utilized postpolymerization modifications of specifically designed precursor polymers to create polymers with identical chain length but different molecular mass. To demonstrate the universality of the findings, two different polymer backbones were utilized poly(styrene) and poly(norbornene imide alkyne)-whose molecular weights were altered by bromination and removal of pendant triisopropylsilyl protecting groups, respectively. Solutions of the respective polymer pairs were subjected to pulsed ultrasound at 20 kHz and 10.4 W/cm(2) in order to investigate the chain scission trends. The effects of cleavage and sonochemical treatments were monitored by size exclusion chromatography. In both series, experimental data and calculations show that the molecular weight reduction upon sonication is the same for polymers with the same contour length.