Water accessibility of single wall carbon nanotubes (SWCNTs) can be monitored by resonant Raman spectroscopy (RRS) (see figure and cover) of the radial breathing modes of bile salt-solubilized CNTs. The RRS features of empty and water-filled CNTs can be very well resolved, and vibrational and electronic shifts and broadenings have been accurately determined. The results also lead to a simple ratiometric method to quantify tube opening under various chemical and mechanical treatments.