The effect of surfactant in dispersing single-walled carbon nanotubes (SWCNTs) to form conducting films were examined using UV-Vis-NIR absorption spectroscopy. The distinctive sub-band transition peaks of sodium dodecyl sulfate (SDS)-dispersed SWCNTs in water were severely suppressed in the film due to the presence of residual SDS interacting strongly with SWCNTs. The recovery of electronic structures of SWCNTs could be monitored systematically using a simple acid treatment through the peak intensities and peak positions of the corresponding transition energies of absorption spectrum. The changes in electrical and optical conductivity of SWCNT film before and after acid treatment are discussed.