Electrochemistry, FTIR-external reflection spectroscopy (FTIR-ERS), and scanning tunneling microscopy (STM) were used to study the ability of n-alkanethiol self-assembled monolayers (SAMs) to protect Au from corrosion in aqueous Br-solutions. The thickness and terminal functional group of the SAMs were varied to determine which factors lead to the greatest corrosion passivation. SAMs prepared on Au from the following molecules were studied : HS(CH2)(15)CH3, HS(CH2)(11)CH3, HS(CH2)(10)COOH, and HS(CH2)(11)OH. The data reveal that, prior to corrosion, the SAMs are crystalline and highly ordered but that afterward they are disordered and oriented parallel to the surface plane. The results show that, for SAMs containing the same terminal functional group, corrosion resistance increases as the SAM thickness increases. For SAMs that are equal in thickness but contain different terminal functional groups, the end groups resulting in the most corrosion resistance follow the order OH > COOH > CH3. The hydrophilic SAM-modified Au surfaces corrode smoothly in a layer-by-layer fashion while the methyl-terminated, hydrophobic SAM-modified surfaces undergo localized corrosion(pitting). Since hydrophilic SAMs are generally more defective than methyl-terminated SAMs but provide the Au more protection, we conclude that initial SAM defectiveness is not a reliable predictor of its barrier properties.