This paper seeks to extend the extant empirical evidence regarding asymmetric adjustment to equilibrium of short and long interest rates. Using an adaptation of the exponential smooth transition model to allow for sign asymmetry in the transition function, we show that equilibrium reversion exhibits two broad characteristics. First, small deviations are random, while large deviations are reverting. Second, deviations that arise when the long rate exceeds the short rate are characterised by quicker reversion than the opposite case. These results are consistent with the effects of arbitrage and central bank intervention. Finally, forecasting exercises support this model over alternate linear and non-linear specifications.