The normal translational energy (E-i) and surface temperature (T-s) dependence of the initial D-2 sticking probability (s(0)) on Pt{100} has been measured using molecular beam techniques. On the hex phase s(0) is found to decrease sharply with E-i between 5 and 10 meV, and to increase more gradually over the range 20 less than or equal to E-i less than or equal to 400 meV. This is interpreted as dynamical steering at low incident energies, giving way to weak translational activation at higher energies. The variation of the barrier to dissociation (E-a) across the unit cell gives rise to screening of dissociation with the impact parameter. There is less variation of E-a across the smaller (1X1) unit cell, leading to a stronger E-i dependence on this surface. On the hex phase s(0) has been measured over a wide T-s range, from 150 to 1100 K. Weak thermal activation of dissociation is observed. This is well explained by a roughening model in which s is assumed to be proportional to the linear thermal displacement of surface metal atoms. The model is supported by density functional theory calculations of potential energy surfaces for a modelled rough surface. The absolute value of s(0) under conditions of zero-point motion is estimated as 0.03 at E-i=69 meV and 0.06 at 342 meV.