Several suspensions containing different types of multi-walled carbon nanotubes (MWCNTs) with different dispersion states were prepared, including homogeneous solutions of MWCNT/polymer composites in an organic solvent. The ultraviolet–visible (UV–vis) absorption spectra from 350 to 750 nm were measured, which showed that the absorbance agreed well with Beer’s law and an apparent absorption coefficient was calculated. For mono-dispersed MWCNTs, the apparent absorption coefficient was independent of the diameter and length of the MWCNT. The coefficient can be used to determine the concentration of well-dispersed MWCNTs. For poly-dispersed MWCNTs, large dense agglomerates of MWCNTs caused an obvious decrease in the apparent absorption coefficient. A simple method for characterizing the concentration and dispersion state of MWCNTs in suspension or polymer composites quantitatively by spectrophotometry was developed.

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