Intermolecular noncovalent interaction is very important for understanding structures and properties of molecular complexes and clusters.Electrospray ionization mass spectrometry（ESI-MS） has demonstrated its powerful utility in the detection and research of weakly bound complex ions,including amino acids,peptides and proteins.In this paper,the noncovalent complex ions of arginine and tryptophan with phosphoric acid were investigated by ESI-MS method.The experiments were performed on a Fourier transform ion cyclotron resonance(FT ICR) mass spectrometer. The results showed that for both systems,wide varieties of noncovalent complex ions were generated and detected in the mass spectra.Most of these complex cations are singly or doubly charged,including both units of amino acid and phosphoric acid.The differences among different systems are also reflected by the distributions of the formed complex ions.