  The Standard-Model Higgs boson with mass $ M_H >> 2M_Z $ decays almost
exclusively to pairs of $W$ and $Z$ bosons. We calculate the dominant two-loop
corrections of $ O( G_F^2 M_H^4 )$ to the partial widths of these decays. In
the on-mass-shell renormalization scheme, the correction factor is found to be
$ 1 + 14.6 % (M_H/TeV)^2 + 16.9 % (M_H/TeV)^4 $, where the second term is the
one-loop correction. We give full analytic results for all divergent two-loop
Feynman diagrams. A subset of finite two-loop vertex diagrams is computed to
high precision using numerical techniques. We find agreement with a previous
numerical analysis. The above correction factor is also in line with a recent
lattice calculation.
