The rates of isomerisation of the equilibrium XO-trans⇌ XO-cis in pure liquid furan-and thiophen-2-carbaldehydes have been measured by using the ultrasonic-pulse technique. In furan-2-carbaldehyde these rates were found to be complementary to a recent n.m.r. study. From the Eyring rate equation and the temperature dependance of the exchange rates the activation enthalpies for the XO-cis→ XO-trans reaction are of the order 10–11 kcal./mole for furan- and thiophen-2-carbaldehydes. In furan-2-carbaldehyde these energy barriers are compared to those found in spectroscopic studies of torsional frequencies. The Arrhenius rate parameters for furan-2-carbaldehyde (E= 11·5 ± 0·5 kcal./mole and A= 2 ± 0·8 × 1013 sec.–1) were derived from the combined ultrasonic and n.m.r. data. These kinetic parameters are in reasonable agreement with a simple mechanism for internal rotation.