While a few singular reports have demonstrated carbon nanotube (CNT) transistors with subthreshold swings (SS) close to the theoretical limit (60 mV/decade), the majority of devices have more than double the target swing. Here, we show that a low temperature lanthanum oxide dielectric is able to yield a consistently and reproducibly low SS, with an average of 73 mV/decade and a low of 63 mV/decade. This LaOx film is characterized using medium energy ion scattering and shown to be scalable down to 3.5 nm with minimal leakage and a variation in swing of only ±13%. With interface traps playing a dominant role in the switching behavior of CNT transistors, these results reveal the existence of dielectrics with more favorable interfacial characteristics for nanotubes that yield low SS devices. © 2013 American Institute of Physics.