Recent results from a measurement campaign studying the isomerism in neutron-deficient Tl isotopes are presented. The measurements make use of a nuclear spectroscopy setup coupled to the high-resolution Penning- trap mass spectrometer ISOLTRAP at CERN???s radioactive ion-beam facility ISOLDE. The mass values of 190,194Tl are improved and a mass-spin-state assignment is carried out. An additional mass measurement of the grandparent nuclide 198At allows the deduction of the spin-state ordering in 190Tl. As a result, the excitation energies of the isomers in both Tl isotopes are determined for the first time to Eex(194Tl) = 260(15) keV and Eex(190Tl) = 89(12) keV. Furthermore, this allows anchoring of the ground-state and isomer masses of 194Bi, 202Fr, and 206Ac, which are linked by two independent $α$-decay chains.