The photooxidation of a range of methyl terminated straight chain alkylthiol (CH3(CH2)(n-l)SH) self-assembled monolayers (SAMs) on Au has been investigated using X-ray photoelectron spectroscopy (XPS). The photooxidation process has been found to involve the oxidation of the thiolate group to a sulfonate by an active O-2 species (possibly singlet oxygen). The rate of photooxidation was found to vary strongly with alkyl chain length and has been correlated with the amount of disorder within the monolayers. Short-chain SAMs (n less than or equal to 8) have the highest disorder and oxidize the fastest, with the rate of oxidation increasing sharply with decreasing n, while close packed long-chain SAMs (n greater than or equal to 12) oxidize much more slowly, with the rate of oxidation decreasing slowly with increasing n.