We quantitatively demonstrate the importance of high purity for the application of single wall carbon nanotubes (SWCNTs), materials solely composed of one surface, by examining the effects of carbon impurities on the electrical, thermal, and mechanical properties of both as-grown SWCNT forests and processed buckypaper. While decreases in properties were expected, our results showed the extreme sensitivity of SWCNT properties to carbonaceous impurities either through scattering in the individual SWCNTs or an inhibition of the ability to form inter-SWCNT junctions. Each property showed a nonlinear decrease (as high as 40%) with the addition of low levels of carbon impurities (∼15 wt%), which demonstrates that purity is as important as the crystalline structure.