The separation of gas molecules and water vapor has become increasingly important for electronic, energy, and environmental systems. Here we demonstrate a new mechanism of enhanced condensation, agglomeration, and rejection of water vapor by superhydrophobic aligned multiwalled carbon nanotubes with the intertube distance of 73 nm, channel aspect ratio of ~5.5 × 10(4), and tortuosity of 1.157. The array with the characteristic channel dimension some 300 times greater than the target molecule size effectively suppressed water molecular transport at room temperature with the selectivity as high as ~2 × 10(5) (H(2)/H(2)O). The flow through the interstitial space of nanotubes allowed high permeability of other gas molecules (2.1 × 10(-9) to 3.8 × 10(-8) mol · m/m(2) · s · Pa), while retaining high selectivity, which is orders of magnitude greater than the permeate flux of polymeric membranes used for the water-gas mixture separation. This new separation mechanism with high selectivity and permeate flux, enabled by the unique geometry of aligned nanotubes, can provide a low-energy and cost-effective method to control humidity.