Electrophoresis is used to separate carbon nanotubes from other by-products during CVD growth from iron catalyst particles, using C2H2 as the carbon feedstock. Carbon nanotubes are trapped by electric fields with higher efficiency than other carbon-containing products. The structure and yield of the carbon nanotubes depends critically on the gas parameters and applied electric field used in the novel reaction chamber. A higher yield of single-walled carbon nanotubes (SWNTs) can be obtained on electrodes in comparison to exhaust filters. The results indicate that the NTs produced by thermal CVD in the gas phase are negatively charged.