In this paper we describe the use of a quartz crystal microbalance (QCM) to follow the progress of a reaction between solution species and specific electrodeposited polymers on an electrode surface, as might be appropriate during immobilisation stages in biosensor production. Two classes of modification are illustrated: firstly, the (reversible) chelation of Ni2+ by carboxylic acid groups pendant to a poly(pyrrole) chain, which may subsequently be used to bind a second molecule; and secondly, the (irreversible) substitution reaction at a polymer bound activated eater by a bifunctional solution phase species bearing both nucleophilic amine and metal ligating thiol groups.