The large amount of hysteresis and threshold voltage variation in carbon nanotube transistors impedes their use in highly integrated digital applications. The origin of this variability is elucidated by employing a top-coated, hydrophobic monolayer to passivate bottom-gated devices. Compared to passivating only the supporting substrate, it is found that covering the nanotube channel proves highly effective and robust at improving device-to-device consistency - hysteresis and threshold voltage variation are reduced by an average of 84 and 53%, respectively. The effect of gate and drain-source bias on hysteresis is considered, showing strong dependence that must be accounted for when analyzing the effectiveness of a passivation layer. These results provide both key insight into the origin of variability in carbon nanotube transistors and a promising path for resolving this significant obstacle. © 2012 American Chemical Society.