We study spectra generated by Comptonization of soft photons by cold electrons radially free-falling onto a black hole .
We use a Monte Carlo method involving a fully relativistic description of Comptonization in the Kerr space-time .
In agreement with previous studies , we find that Comptonization on the bulk motion of free fall gives rise to power-law spectra with the photon index of \Gamma \ga 3 .
In contrast to some previous studies , we find that these power-law spectra extend only to energies \ll m _ { e } c ^ { 2 } .
We indicate several effects resulting in generic cutoffs of such spectra at several tens of keV , regardless of any specific values of physical parameters in the model .
This inefficiency of producing photons with energies \ga 100 keV rules out bulk motion Comptonization as a main radiative process in soft spectral states of black-hole binaries .
The normalization of the power law ( below the cutoff ) with respect to the peak of the blackbody emission of the surrounding disc is typically very low , except for models with an overlap between the disc and the plasma , in which case the spectra are very soft , \Gamma \ga 4 .