A characteristic feature of the high-temperature superconductors is the existence of a chemical composition that gives a maximum transition temperature, T-c, separating the so-called under-doped and over-doped regimes(1,2). This behaviour is thought to be universal for high-temperature superconductors. In practice, there are only a few high-T-c compounds for which the composition can be varied continuously throughout the entire doping range. Here we report a study of correlations between structure and T-c in a compound with the ＇123＇ structure in which both the under-doped and over-doped regimes can be accessed We observe a clear scaling between T-c and the buckling of the copper oxide planes;both go through a maximum at the same oxygen composition (and hence doping level), so implying a common origin. Previous work has shown that, for a fixed chemical composition, increased CuO2 plane buckling: lowers the transition temperature(3-11). Thus the observation of a maximum in the buckling at the maximum T-c indicates that, as the composition is changed to increase T-c, there is a structural response that competes with superconductivity.