The effects of charge transfer induced by analyte molecule adsorption on the sensing properties of single-walled carbon nanotube (SWCNT) fiber optic chemical nano-sensors has been investigated. Experimental evidence indicates that extrinsic fiber optic Fabry–Perot (FP) interferometers incorporating nano-scale sensitive layers of SWCNTs and cadmium arachidate exhibit responses of opposite sign on exposure to electron donating (xylene and ethanol vapors) or accepting (NO2) analytes, at room temperature. This reveals the strong influence of the electrical nature of the adsorbed species on the optical properties of carbon nanotube overlays. To take account of this influence, the plasma optic effect has been considered, which allows one to relate the modulation of the optical properties of sensitive overlays to the changes of carrier concentration. The results reveal that in analogy with resistive sensors based on …