The substituted-cysteine accessibility method (SCAM) provides an approach to identifying the residues in the membrane-spanning segments that line a channel, transporter, or binding-site crevice. SCAM can also be used to determine differences in the structures of the membrane-spanning segments in different functional states of the proteins, to map electrostatic potential in the membrane-spanning domains, and to size a channel or binding-site crevice. The protocol in this unit describes the use of SCAM to map the binding-site crevice of a G-protein coupled receptor (GPCR) which binds ligand within the transmembrane portion of the receptor.