Mid-infrared spectra of solutions in liquid argon, krypton, and xenon of methyl nitrite, which occurs as a mixture of cis and trans conformers, have been investigated. In liquid argon and in liquid krypton at temperatures below 150 K, the rate of conformational equilibration is found to be negligible, while at temperatures above 160 K in liquid krypton, and in liquid xenon, the equilibration is near-instantaneous. The standard enthalpy difference DeltaH degrees between the conformers in liquid krypton was measured in the 165-205 K interval to be 3.02(9) kJ mol(-1), with the cis as the more stable conformer. Starting from solutions containing nonequilibrium. populations of the conformers, the conformational equilibration was studied as a function of time at five different temperatures between 150 and 160 K in liquid krypton. From these kinetic data, the extinction coefficient ratios for the nu (3) and nu (8) conformational doublets have been determined to be 1.2(2) and 4.8(8), respectively. The extinction coefficient ratio for nu (8) was combined with the value for DeltaH degrees to yield the value of 5.1(2) J K-1 mol(-1) for the standard conformational entropy difference DeltaS degrees. The kinetic data have also been used to calculate the enthalpy and entropy of activation for the interconversion from trans to cis conformer, and were found to be 44.0(14) kJ mol(-1) and 16(9) J K-1 mol(-1), respectively. The results are discussed in light of literature data and in light of perturbation Monte Carlo calculations on solvation enthalpy and entropy.