Dissociative adsorption of H-2 from a high-flux supersonic molecular beam on flat and vicinal Si(001) surfaces was investigated by means of optical second harmonic generation (SHG). The initial sticking coefficients for terrace adsorption varied between 10(-8) and 10(-4). They revealed a strongly activated dissociation process, both with respect to the kinetic energy of the incident molecules (70 meV less than or equal to E-kin less than or equal to 380 meV) and the surface temperature (440 K less than or equal to T-s less than or equal to 670 K). The results indicate that dynamical distortions of Si surface atoms can lower the effective adsorption barriers from 0.8 +/- 0.2 eV to almost negligible values. Previously proposed defect-mediated processes can be ruled out as a major adsorption channel.