The kinetics of ultrasonic degradation has been investigated for dilute polystyrene solutions in decalin. The increase in the scission rate constant (K) with initial molecular chain length was systematically investigated over 2 decades of molecular weight by using narrow polymer fractions with M = 50 x 10(3) - 8.5 x 10(6) Da. An empirical relation of the form K similar to (M - M-lim)(1.9), with M-lim = 30 000, was found for 50 x 10(3) < M < 2.2 x 10(6). The rate constant becomes independent from M at higher molecular weight. This complex behaviour is rationalized by establishing a parallelism between transient elongational flow and ultrasonic degradation.