This paper studies the dynamic properties of relative commodity prices, especially the Presbisch-Singer hypothesis on the secular decline in these series, using a new family of unit root tests that is based on the Fourier approximation to the underlying trend in the data. The approximation controls for low-frequency variations such as structural breaks, or such as the long swings induced by hypothesized super cycles in the data. Relative to the extant literature, we find considerably more evidence in favor of trend stationarity in relative commodity prices, and relatively limited support for the Prebisch-Singer hypothesis.