A very easy and simple way of filling single-wall carbon nanotubes (SWNTs) at room temperature in open air was discovered by soaking as-prepared SWNT materials in a mixture of a transition metal oxide in an inorganic acid (CrO3 in HCl). The filling of SWNTs was found to start just after three hours of treatment and to increase with the treatment time up to two days. Both X-ray energy-dispersive spectroscopy (X-EDS) and electron energy loss spectroscopy (EELS) suggest that the filling compound is a chromium-containing material, probably CrO3. Filled SWNTs are stable in vacuum under electron beam (using conventional imaging conditions), in air and in polar solvents like water, alcohol, and acetone.