The hyperfine interactions of single and of exchange-coupled pairs of Ir⁴⁺ ions in (NH₄)₂PtCl₆ have been investigated by ENDOR methods at 4 °K. Recent investigations of electron transfer between magnetic ions have suggested that a measurable difference may occur. Experimentally, however, the difference is found to be less than 0·15%. The hyperfine constants are determined to be negative, leading to a value K = 0·85±0·4 for the core polarization constant for 5d electrons. The nuclear dipole moments of ¹⁹³Ir and ¹⁹¹Ir are determined to be ¹⁹³μ = +0·162±0·006 nuclear magnetons and ¹⁹¹μ = +0·146±0·008 nuclear magnetons, in agreement with recent measurements by nuclear magnetic resonance in the pure metal. A hyperfine anomaly of ₁₉₁¹⁹³Δ = (0·20±0·30)% is found. The ratio of the nuclear quadrupole moments is determined to be ¹⁹³Q:¹⁹¹Q = 0·92±0·09.