We report on the effects of monolayer (ML) thick skin-like octadecanethiol (ODT, CH3[CH2](17)SH) on type-II InAs/GaSb MWIR photodetectors. Circumventing the ageing effects of conventional sulfur compounds, we use ODT, a self-assembling, long molecular chain headed with a sulfur atom. Photodiodes coated with and without the self-assembled monolayer (SAM) ODT were compared for their electrical and optical performances. For ODT-coated diodes, the dark current density was improved by two orders of magnitude at 77 K under -100 mV bias. The zero bias responsivity and detectivity were 1.04 A W (1) and 2.15 x 10(13) Jones, respectively, at 4 mu m and 77 K. The quantum efficiency was determined to be 37% for a cutoff wavelength of 5.1 mu m.