The truncation of Volterra series expansions is studied using the output frequency characteristics of nonlinear systems to develop a new algorithm for determining the terms to include in a Volterra series expansion. The results show the influence of both the generalized frequency response functions and properties of the input spectra on the significance of individual terms in the series. The effectiveness of the proposed method is demonstrated using simulation studies including the analysis of a single degree of freedom mechanical oscillator. Nonlinear system analyses using Volterra series theory must always be based on a truncated Volterra series description. The present study provides an effective strategy for determining which terms to include in the analysis of practical nonlinear systems based on Volterra series models.