Two series of iminoxy radicals, R1C(=O)C(=NO)R-2 and R1CH2C(=NO)R-2, were studied using DFT calculations and EPR spectroscopy. The role of stereochemistry in long-range spin density distribution was discussed in terms of the configurational and conformational effects. To identify the geometric isomers (E or Z) of the iminoxyls obtained in solution as well as in the matrices of zeolite and gamma-irradiated oximes, the hyperfine parameters derived from the EPR spectra were compared with those predicted by DFT. The spectra indicate that C6H5COC(NO)H and CH3COC(NO)H undergo isomerization in the solid oxime. The contribution of a particular isomer, dependent on temperature, corresponds to the calculated relative energies of the isomers. (C) 2004 Elsevier B.V. All rights reserved.