AbstractThe reaction H2→ 2 H (dissolved) on thin Pd and Pd‐Ag foils (2.5–5 μm) was studied, with the aid of resistance measurements, in a flow apparatus at temperatures in the range 20–150°C. The rate determining step is the dissociation of the H2‐molecule on the metal surface. It is assumed that in the formation of the activated complex one atom (proton) already enters an interstitial site in the lattice. For very low hydrogen concentrations xHin α‐phase Pd and in Pd‐Ag alloys, where Sieverts law applies, the following rate law is validThe rate law shows that there is a segregation equilibrium between dissolved atoms and atoms in the interstitial reaction sites. For α‐phase Pd the enthalpy of this segregation is ‐1.9 kcal/mol H, the activation energy of the forward reaction is 6.8 kcal and of the backward reaction 11.5 kcal, which yields the reaction enthalpy ‐4.7 kcal/mol H.In a range of higher hydrogen concentrations the forward reaction rate is accelerated by a factor exp(gxH). This leads to the decreased slope of the isothermp H 21/2vs.xH, that is known from thermodynamic studies. The acceleration is ascribed to the lattice expansion caused by the dissolved atoms. After the d‐band of the metal phase is filled by electrons from hydrogen (and silver), the forward reaction rate decreases with increasingxHand the backward reaction rate increases exponentially. This leads to the known steep rise of the isothermp H 21/2vs.xH in the ß‐phase region or in the