Renewed interest in absorption of H and D into transition metals has been stimulated by current work on metal-hydride anodes for rechargeable batteries and by the intense interest in recently studied anomalous phenomena associated with absorption of D into Pd. One principal problem in this field is how site-blocking elements (SEE), also called "catalyst poisons" (P), co-adsorbed with H on transition metal H hosts, usually substantially promote the absorption of H into a number of such metals, yet such poisons are known to inhibit cathodic H-2 evolution and diminish surface coverage by H, the intermediate in the latter process and the source of the absorbed H. These effects of catalyst poisons in promoting H absorption into host metals are examined in terms of effects of competitive adsorption between the poison and H at the surface. Conventional ideas about these effects are inconsistent with information on diminutionn of H coverage caused by competitive adsorption of poisons. Consideration of various possibilities indicates that the enhancement of H absorption by SBEs can arise only on account of direct interaction effects and long-range indirect electronic charge-density effects. Numerical simulation calculations are given on the competitive chemisorption of H vis a vis a poison; P, in relation to the configurational contribution to the Gibbs energy of the adlayer, H + P. Numerically derived, three-dimensional diagrams are presented to illustrate these effects. The role of lateral interaction effects among adsorbed H and P species in the thermodynamics and kinetics of H absorption is also examined.