[en] The 1.87 hours 543.7 keV (7⁺) isomeric state in ¹⁹⁸Tl is produced via ¹⁹⁷Au(α, 3n)sup(198m)Tl (Esub(α) = 35 MeV) reaction. The total conversion coefficient of 260.9 keV (7⁺ →3^-) is determined for the first time by the intensity balance method. The value of αsub(T)(260.9 keV) is found to be 40.1 +- 8.6 which is in good agreement with the theoretical value of Hager and Seltzer for pure M4 transition. The gamma transition probability of the 260.9 keV (M4) is calculated using the present value of αsub(T) and compared with the single-particle estimate