The intense line in Mims and Davies electron-nuclear double resonance (ENDO
R) spectra due to the hyperfine interactions of an unpaired electron with d
istant matrix nuclei is shown to originate from a simultaneous inversion of
a large number of nuclear spins by a radiofrequency pulse. Theoretical exp
ressions describing the matrix ENDOR effect are derived and verified experi
mentally. (C) 1998 Academic Press.