The recent commercialization of single-walled carbon nanotubes (SWNT) with controlled chirality distributions has created new opportunities for producing SWNT materials with tailored electrical properties. However, there has 100 been relatively little research on understanding the effects of chirality distribution and SWNT purity on the two main determinants of nanocomposite mechanical properties: dispersion microstructure and nanotube-resin interactions. To establish a framework for comparing dispersion and interactions, we investigated the viscoelasticity of four South West Nano Technologies' SWNT products: a low and high purity semiconducting grade and a low and high purity metallic grade. Optical microscopy of the dispersions did not show any significant differences between the SWNT types. However, analysis of the dispersions' viscoelastic properties revealed the difference in dispersion microstructure and the relative strength of SWNT resin interactions. While all four products had a similar percolation threshold, the concentration of non-SWNT carbon impurities had a greater effect than chirality on viscoelastic properties and the relative strength of resin SWNT interactions.