The application of many of the well-known methods of inversion to rainfall excess and direct runoff data often produces finite-period unit hydrographs whose ordinates are distorted by high frequency oscillations. A simple extension of the basic algorithm for the harmonic method of deriving the finite-period unit hydrograph provides a means of smoothing such oscillations. The modified method has computational advantages over the use of moving average smoothing schemes applied to the derived unit hydrograph.