We investigated the stabilities of poly(ethylene glycol)-terminated self-assembled monolayers (PEG-SAMs) and quantitatively evaluated their inhibition of nonspecific protein adsorption on Au substrates. PEG-SAMs with 16-carbon alkyl chains bound stably to the Au substrate and inhibited nonspecific protein adsorption, in contrast to PEG-SAMs with 11 or fewer carbon alkyl chains, as demonstrated by 2 months' storage in a protein mixture at 37 degrees C.