The IR spectrum of the pyrolysis product of azoisobutane has been shown to yield the matrix-isolated tert-butyl radical. In support of this its spectrum has been simulated by a normal-coordinate calculation combined with a calculation of the infrared intensities of all vibrations by a modified INDO open-shell procedure. For comparison, the frequencies and infrared intensities of the isobutane molecule have been calculated by using the INDO closed-shell option and the force field of Snyder and Schachtschneider. Both observed spectra are well reproduced, showing that the method of simulation is working satisfactorily for molecules with both closed and open shells. The pyramidal deformation vibration of the tert-butyl radical is calculated to have a low intensity and to have a band center at 158 cm-1, below the limit of the observed matrix spectrum at 250 cm-1. This is in contrast to the same vibration in the ethyl radical which is calculated and observed to be the most intense vibration in the spectrum at 540 cm-1, but it is in accordance with the expectations concerning the properties of primary, secondary, and tertiary radicals. © 1984 American Chemical Society.