In higher plants, both photosystem I (PSI) and II (PSII) consist of membrane-embedded proteins that contain more than one transmembrane alpha helix. PSI is a multiprotein complex consisting of a core complex of thirteen proteins surrounded by four different types of light harvesting antenna proteins. Up to now, the protein components of both photosystems have been characterized by SDS-PAGE and/or immunoblotting and, therefore, identification made only on the basis of electrophoretic mobility, which is sometimes not sufficient to discriminate between individual membrane proteins. This is also complicated by the fact that some proteins, such as the antenna proteins, have almost identical molecular mass and amino acid sequence, making it difficult to identify and ascertain the relative stoichiometry of the proteins. In this paper, we report the complete resolution of the antenna proteins and most of the core components of PSI from spinach, together with the identification of proteins by molecular mass, successfully deduced by the combined use of HPLC coupled on-line with a mass spectrometer equipped with an electrospray ion source (ESI-MS). The proposed RP-HPLC-ESI-MS method holds several advantages over SDS-PAGE, including better protein separation, especially for antenna proteins, mass accuracy, speed, efficiency, and the potential to reveal isomeric forms. Moreover, the molecular masses determined by HPLC-ESI-MS are in good agreement with the molecular masses of the individual components calculated on the basis of their nucleotide-derived amino acid sequences, indicating an absence of post-translational modifications in these proteins. It follows that if the method proposed is useful for these highly hydrophobic proteins, it may be of general use for any membrane proteins, where the presence of detergent for solubilization may compromise their characterization.