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One-phonon states of vibrational nuclei with mixed proton–neutron symmetry have been observed throughout the nuclear chart besides the mass A ≈ 200 region. Very recently, it has been proposed that the 2+2 state of ²¹²Po is of isovector nature. This nucleus has two valence protons and two valence neutrons outside the doubly-magic ²⁰⁸Pb nucleus. The stable isotope ²⁰⁴Hg, featuring two valence-proton and valence-neutron holes, with respect to ²⁰⁸Pb, is the particle-hole mirror of ²¹²Po. In order to compare the properties of low-lying isovector excitations in these particle-hole mirror nuclei, we have studied ²⁰⁴Hg by using the projectile Coulomb-excitation technique. The measured absolute B(M1; 2+2 → 2+1 )strength of 0.20 (2)μ2 N indicates that the 2+2 level of ²⁰⁴Hg is at least the main fragment of the 2+1,ms state. For the first time in this mass region, both lowest-lying, one-quadrupole phonon excitations are established together with the complete set of their decay strengths. This allows for a microscopic description of their structures, achieved in the framework of the Quasi-particle Phonon Model.