The infrared spectra of CH3CH2, CH3CD2, CD3CH2, and CD3CD2 have been observed in argon matrices by photolysis of the corresponding dipropionyl peroxides. The spectra of the isotopic species coupled with detailed ab initio calculations have provided the basis for an assignment. The analysis has revealed the spectral features that are clearly a manifestation of the unpaired electron. © 1982, American Chemical Society. All rights reserved.