Understanding the unique role that Pr hybridization plays in promoting both the suppression of superconductivity and the onset of complex magnetism in high-TC cuprates continues to be a dynamic area of research. Complex magnetic behavior is well known to occur in the (Pr1.5Ce0.5)Sr2Cu2MO10 (M = Nb or Ta) compounds. Previously, two temperature-dependent transitions have been identified through magnetic susceptibility studies for the series of solid solutions (Eu1.5−xPrxCe0.5)Sr2Cu2NbO10 (0.0 ≤ x ≤ 1.5). In this paper, we present new measurements of the Raman-active modes and the magnetic susceptibility for these latter compounds. The data indicate a discontinuity in concentration space at a Pr doping level of x = 1.0, that we interpret as the signature of a subtle phase transition. Copyright © 1999 John Wiley & Sons, Ltd.