The electronic states of carbon nanotubes are one of the most fundamental properties of the nanotubes. We now describe the finding that the band gaps of (n,m)SWNTs are strongly affected by the change in microdielectric environments around the isolated nanotubes. In situ photoluminescence (PL) spectroelectrochemistry of the films containing 15 isolated (n,m)single-walled carbon nanotubes (SWNTs) cast on ITO electrodes in organic solvents including DMSO, acetonitirile, DMF, THF, and chloroform was completed and then the oxidation and reduction potentials, and band gaps (Delta E-electr) of the (n,m)SWNTs in the solvents were determined. We have discovered that the Delta E-electr, of the (n,m)SWNTs become greater as the solvent dielectric constants decreased, which is in sharp contrast to the optical band gaps (Delta E-opt) that show virtually no solvent dependence. Such a strong solvent dependence of the electrochemical band gaps is due to the difference in the solvation energy of the charged SWNTs produced during the electrochemical processes. The Delta E-electr, of both mod types of the SWNTs, mod = 1 and mod = 2, linearly increased versus the reciprocal of the tube diameter, which agrees with the theory. Moreover, the states of the pi-electrons in the SWNTs were evaluated from the dependence of the band gaps on the diameter of the SWNTs. Furthermore, the states of the pi-electrons on the sidewalls of the SWNTs were evaluated using the gamma(0) values, a parameter representing the measure of the stability or the degree of delocalization of pi-electrons in the sidewall of the SWNTs, and revealed that the gamma(0) values of the mod = 1 and mod = 2 SWNTs increased with a decrease in the dielectric constants of the solvents in the range of 38-79. This study has enabled us to understand the essential electronic properties of the carbon nanotubes.