A surface complexation model is used to describe sorption of ytterbium and cesium on the silica surface. The constant capacitance model gives the description of the solid-solution interface chosen for this work. The first step in the modeling consists of extracting the surface acidity constants. The result is: SOH reversible arrow SO- + H+ log Ka(2) = -7.60 +/-0.05. The second step consists of the extraction of surface complexation constants for both ytterbium and cesium, The sorption of the cations is represented as follows: for the ytterbium sorption, 2H(2)O + SOH + Yb3+ reversible arrow SOYb(OH)(2) + 3H(+) log K = -16.2 +/- 0.3 for the cesium sorption, SOH + Cs+ reversible arrow SOHCs+ log K = 2.05 +/-0.05 SOH + Cs+ reversible arrow SOCs + H+ log K = -5.5 +/- 0.1. In the case of cesium, the sorption of sodium is competitive and has to be considered: SOH + Na+ reversible arrow SOHNa+ log K = 2.2 +/- 0.1.