We have measured the temperature dependence of the absorption coefficient and integrated absorption of the liquid nitrogen 2.15-μm (4647 cm -1) 2-0 collision-induced band. The integrated absorption of the liquid is observed to be smaller than that of the gas, and decreases slightly with decreasing temperature. The band gets sharper with decreasing temperature with a half-width proportional to the square root of the temperature. If this temperature-dependent behavior is extrapolated to the nominal surface temperature of Triton, a new estimate of the grain size of N 2 ice on Triton's south polar cap can be made. We employ Hapke scattering theory with previously published IR spectra of Triton to estimate a mean nitrogen grain size of between 0.7 and 3 cm. This results is consistent with results from grain growth rate calculations.