Thermoelectric properties of single wall carbon nanotubes (SWNT) are quite sensitive to gases in contact with the tube walls. This effect makes possible a thermoelectric chemical sensor. Large, reversible swings in thermoelectric power (S), sometimes even involving sign changes in S, have been observed. Even contact of the SWNTs with He and N-2 and H-2 result in easily detectable and reversible changes in S. Smaller, polar alcohol molecules stimulate a large thermoelectric response, although H2O has no effect. For adsorption of six membered ring molecules C6Hn in SWNTs, the large thermoelectric response observed for Benzene (n=6) is seen to decrease as the pi electrons in the molecule are removed, and the coupling between the molecules and the SWNT is thereby reduced. These effects are discussed in terms of the diffusion thermopower for a rope, and a new scattering channel associated with adsorbed molecules.