The specification discloses a method and apparatus for the assay of salicylate (or a derivative thereof) comprising a conducting body having at a surface thereof an enzyme capable of catalyzing the conversion of salicylate (or a derivative thereof) to a catechol whereby said catechol is subject to direct electrochemical measurement as it oxidizes at said surface to generate a current as a measure of the reaction taking place and thereby of the concentration of salicylate at said surface. The method typically comprises the steps of;(a) treating a liquid sample suspected of containing salicylate or a derivative thereof with an enzyme capable of catalyzing the conversion of salicylate or a derivative thereof into a catechol, and,(b) measuring the concentration of catechol in the treated sample by direct electrochemistry.