Abstract: A method for detecting a membrane protein using a desorption electrospray ionisation source coupled to a mass spectrometer comprises: desorbing a membrane protein from a surface on which a sample comprising the membrane protein and a solubilising agent is deposited by applying electrospray to the sample; ionising the membrane protein; and detecting the membrane protein using the mass spectrometer. The method may be used for deriving information about a membrane protein, e.g. its structure or conformation or, where a ligand is present, the stoichiometry and dissociation constant of a complex in which the membrane protein is bound to the ligand.