Single-wall carbon nanotubes (SWNT's), deposited onto an Al substrate from a liquid suspension, have been cleaned by annealing in UHV. The effects of exposing the sample in situ to atomic H (or D) and/or to DMMP [dimethyl methylphosphonate, (CH@sub 3@O)@sub 2@(CH@sub 3@)P=O] were then studied using polarization-modulated infrared reflection-absorption spectroscopy. Atomic H reacts preferentially near strained or defective regions in the nanotube wall to produce a spectrum consistent with alkane-like species (>CH@sub 2@ and CH@sub 3@). Only a small fraction of the >C=C< sites in the nanotube wall react with H, and there is no clear evidence for monohydride (>C(H) C(H)<) species. For DMMP, data were obtained under steady-state conditions in reagent pressures in excess of half the room-temperature vapor pressure. Adsorption occurs via the P=O group with a coverage that depends on the ambient pressure. Varying the DMMP coverage by changing the pressure causes changes in the spectrum that can be related to the strength of the DMMP/SWNT interaction. Pre-adsorbed H is seen to have little or no effect on the subsequent adsorption of DMMP. For DMMP, the molecular features are superimposed on a broad, smoothly-varying background that can be related to adsorption-induced changes in the Drude parameters characterizing the SWNT free-carrier density and scattering lifetime. Data were also obtained during exposure to @super 16@O@sub 2@ or @super 18@O@sub 2@. As in the case of atomic H, evidence is seen for preferential reaction near strained or defective regions, but no vibrational modes of adsorbed O or O@sub 2@ are detected.