The first structure-reactivity relationship for electron-transfer reactions of single walled carbon nanotubes (SWNTs) has been derived and experimentally validated using 4-hydroxybenzene diazonium as a model electron acceptor. The model describes steady-state reaction data using an adsorption-controlled scheme, and electron transfer theories are used to explain the difference in reactivities between different nanotube chiralities. The formalism provides a mechanistic insight into electronically selective reactions. The influence of reagent concentration and external illumination (∼0.764 mW/cm2) on the reaction selectivity is described by the rate model, with quantitative descriptions of the changes in the UV-vis-nIR absorption spectra of nanotubes during reaction. Illumination was shown to decrease the selectivity of the reagent to metallic SWNTs over semiconducting SWNTs. We attribute this to the greater activity of the reagent in solution when exposed to light, resulting in greater extents of reaction for each SWNT and, hence, lower selectivity.