Abstract: Sensor devices for use in assaying for a ligand in a sample are described, the devices comprising: i) a discrete zone ("the measurement zone") on a region of which ("the measurement region") is immobilized directly or indirectly a first specific binding partner for the ligand under assay (or a reagent precomplexed with or capable of forming a complex with a specific binding partner for the ligand under assay), which zone additionally contains in releasable form, a first known amount of an optionally labelled second specific binding partner for the ligand under assay, the second specific binding partner being directed to an epitope of the ligand assay different to the epitope to which the first specific binding partner is directed; and ii) a second discrete zone ("the reference zone") on a region of which is immobilized directly or indirectly a first specific binding partner for the ligand under assay (or a reagent precomplexed with or capable of forming a complex with a specific binding partner for the ligand