  The amplitude of the diphoton decay of the Higgs boson is considered in an
extension of the Standard Model involving heavy vectorlike fermions, which was
recently suggested in the literature in order to explain a possible enhancement
of the h -> gamma gamma decay rate. It is pointed out that generally in such
models the decay amplitude acquires a sizable CP-odd contribution from the
complex phases in the mass matrix of the new heavy fermions. The resulting CP
violation in the diphoton decay can be studied experimentally when both photons
are converted into e^+ e^- pairs by measuring the distribution over the angle
between the planes of the pairs. Such measurement, if feasible, would be of a
great interest on its own, regardless specific models.
