The application of applied external pressure to solutions containing lanthanide(III) complexes has been shown to provide useful information concerning the molecular and electronic structure of the species in solution. Eu(III) complexes are of special interest since the usually sharp excitation and emission transitions may be analyzed to determine relative concentrations of the species present. In this paper we describe the types of information concerning equilibria and excited state processes that may be obtained by applying external pressure, and present results for solutions of the Ca-binding protein calmodulin in which Eu(III) ion is used as a replacement for Ca(II). It is shown that differences in the binding site volumes in calmodulin may be determined, and these are in agreement with the known crystal structure.