Electroless plating of a metal on a dielectric substrate requires the prior deposition of a catalyst such as a Pd-Sn colloid consisting of a metallic Pd core surrounded by a stabilizing layer of Sn ions. The activation step (deposition of the colloid) is usually followed by an acceleration step (removal of excess ionic tin). Adhesion of the deposit to the substrate is improved by mechanical and chemical pretreatment steps. An electrochemical method has been developed for assessing the catalytic activity of Pd-Sn colloids. Hydrogen sorption in the Pd in the colloid can be correlated with catalytic activity, since Pd accessible for the H-sorption reaction is also accessible as the catalyst for the electroless deposition reaction. These conclusions have been confirmed by surface analytical techniques and by functional tests. The efficacy of various accelerating solutions has also been assessed.