Heterogeneous ice nucleation is an important mechanism for cloud formation in the upper troposphere. Recently, pores on atmospheric particles have been proposed to play a significant role in ice nucleation. To understand how ice nucleation occurs in idealized pores, we characterized the immersion freezing activity of various sizes of carbon nanotubes. Carbon nanotubes are used both as a model for pores and proxy for soot particles. We determined that carbon nanotubes with inner diameters between 2 and 3 nm exhibit the highest ice nucleation activity. Implications for the freezing behavior of porous materials and nucleation on soot particles will be discussed.