A single-walled carbon nanotube (SWNT) is often mentioned as one of the strongest materials known. In tension along the tube axis, this statement is correct. However, the tube is “soft” in the radial direction, i.e., deformation or squash modes which give rise to an oscillating elliptical cross section have freq’s in the range 20-30 cm-1. Here, we present results of an in situ electrical transport study (thermoelectric power (S) and resistivity (ρ) of bundled SWNTs exposed to a series of gases (He, Ar,Ne,Kr,Xe;CH4,N2). Unusually strong and remarkably systematic changes in these transport properties are observed as the nanotubes undergo collisions with these atomic and molecular gases. At fixed pressure and temperature, the changes in the transport parameters, i.e., ΔS and Δρ, are observed experimentally to exhibit an ~ M1/3 behavior, where M is the mass of the colliding gas atom (molecule).1 At fixed temperature, ΔS and