Infrared absorption spectra of liquid methyl iodide (CH3I) and its solutions in CCi4, as 2, heptane, benzene, chloroform and deuterated acetone-d6 have been studied. Infrared spectra in the regions 3400-2700, 1700-1 ! 00 and 1000-750 cm-i were fitted by the sum of components, with the form of multiplication of the Lorentzian and Gaussian functions. E-Type bands under investigation (V 4 = 3047, v 5 = 1428, and v 6 = 885 cm-i) were reproduced by the sums of two components: the narrower (n) and the broader (b) ones. A different temperature behaviour of the components has been found: the integrated intensity of the narrower component (I,) decreases with the temperature, while the intensity of the broader one (Ib) increases. The narrower components of v5 and v 6 were attributed to CH3I molecules moving according to the orientational diffusion mechanism; the broader ones were attributed to molecules, freely rotating about the C3v axis. Some additional mechanism (probably the interactions between CH stretching vibrations with single particle and collective motions of molecular dipoles) was proposed to play a part in forming the v4 bandshape. The enthalpy difference between freely rotating molecules and those moving via an orientational diffusion mechanism (AH) have been determined by the slopes of the dependencies of ln(l,/Ib) upon T-t: AH = 0.8 + 0.1 kcal mol-t. The temperature behaviour of fin has been studied in the 210-340K temperature range, and Rakov's approach has been used to determine the activation enthalpy (AH*) and entropy (AS*) of parallel orientational diffusion in the pure liquid: AS*=-4.5+0.2calmol 1K-J, AH*=0.1+0.1kcalmol J. The CH3 stretching range was found to be strongly affected by a solvent. Total integral absorption coefficients of vj and v4 bands increase two-fold when going from CC14 to acetone-d 6 solution, while 6 n values decrease by 3-9 cm i The observed effects were explained in terms of the existence of complexes with weak ICH 3 .. โข acetone hydrogen bonding. The strength of the hydrogen bonding was characterized by enthalpies of specific interaction ---int.AHCH~l/s (sp.). These values were estimated by the "intensity rule": ---int.AHCH~l' s (sp.) = 0.13 kcal mo1-1 for self-association in pure CH31 and 0.4 kcal mol -~ for solution in acetone-d6.