SWNTs were synthesized in high yield in the presence of Ce/Ni as catalyst. Raman and SEM results show that a fraction of the tubes are chiral and have small diameters (the snallest one is 0.79 nm), indicating that Ce plays an important role in the formation. Acid treatment is effective in eliminating impurities but changes the morphology and the electric resistance of mats with increasing treatment time. The resistance of as-grown and short time acid-treated mats follow a 2D VRH behaviour while mats treated for a long time follow a 3D model. The results indicate that hopping conduction occurs on bundles or on tubes and that there are intrinsic links between tubes in as-grown mats.