Adsorption of telechelic polymers (hydrophobically end-capped poly(ethylene oxide)) on charged colloidal particles modifies the characteristics of the electrical double layer. These interfaces were found to be able to protect colloids against aggregation or to induce colloid destabilization depending on the molecular weight and type of surface coating. Colloid stability is observed when the polymer layer is thinner than the electrical double layer and when the adsorbed polymer adopts a tethered brush structure. Aggregation of coated latex: particles induced by formation of interparticle pol mer links is observed when (i) the distance of the random walk of the dangling paraffinic end group is larger than the thickness of the electrical double layer and (ii) the adsorbed polymer brush adopts a loopy structure. Aggregation is attributed to adsorption of hydrophobic end chains on two different latex particles.