On the basis of lattice calculations, we require the existence of a deconfined quark matter region (0 < as < 1) beyond the hadron gas phase, which goes asymptotically into the ideal quark–gluon plasma domain, in the phase diagram of nuclear matter. We consider empirically the dynamics of this region in terms of the order parameters and mass-scaled partition functions and derive an EoS. Then, the strange-quark chemical potential is expressed in a functional form of the temperature and light-quark chemical potential and its variation throughout the 3-region phase diagram is studied. We propose the change of the sign of the strange-quark chemical potential, from positive in the hadronic region to negative beyond, to be a unique, concise and well-defined indication of the quark-deconfinement phase transition in nuclear matter. Analysis of the nucleus–nucleus collision data from AGS and SPS is presented giving strong support to our proposal.