MOLECULES that perform logic operations are prerequisites for molecular information processing and computation. We and others have previously reported receptor molecules that can be considered to perform simple logic operations by coupling ionic bonding or more complex molecular-recognition processes with photonic (fluorescence) signals: in these systems, chemical binding (the 'input') results in a change in fluorescence intensity (the 'output') from the receptor. Here we describe a receptor (molecule (1) in Fig. 1) that operates as a logic device with two input channels: the fluorescence signal depends on whether the molecule binds hydrogen ions, sodium ions or both. The input/output characteristics of this molecular device correspond to those of an AND gate.