  We report on high-field electron spin resonance studies of two iridium
hexahalide compounds, (NH$_4$)$_2$IrCl$_6$ and K$_2$IrCl$_6$. In the
paramagnetic state, our measurements reveal isotropic $g$-factors $g = 1.79$(1)
for the Ir$^{4+}$ ions, in agreement with their cubic symmetries. Most
importantly, in the magnetically ordered state, we observe two magnon modes
with zero-field gaps of 11.3 and 14.2 K for (NH$_4$)$_2$IrCl$_6$ and
K$_2$IrCl$_6$, respectively. Based on that and using linear spin-wave theory,
we estimate the nearest-neighbor exchange couplings and anisotropic Kitaev
interactions, $J_1/k_B=10.3$ K, $K/k_B=0.7$ K for (NH$_4$)$_2$IrCl$_6$, and
$J_1/k_B=13.8$ K, $K/k_B=0.9$ K for K$_2$IrCl$_6$, revealing the
nearest-neighbor Heisenberg coupling as the leading interaction term, with only
a weak Kitaev anisotropy.
