The results of quantitative and systematic studies of the dependence of two-pulse spin-echo intensity on the pulse widths and of the Davies ENDOR response on the width of the preparation pulse, tp are reported. A formula that describes the Davies ENDOR response as a function of the selectivity parameter, ηη = Antp is presented. These measurements, which employ a variety of small molecules and metalloenzymes, not only provide information about the Davies ENDOR technique, but also give a quantitative description of the recently described POSHE method for hyperfine selection of heteronuclear pulsed ENDOR spectra through manipulation of pulse widths in the Davies ENDOR sequence. This procedure involves suppression of weakly coupled proton pattern and optimization of strongly coupled heteronuclei signals. It is demonstrated with single-crystal data for Cu(II)-doped Zn(glycinato)2 and frozen solution data for the blue copper protein Pseudomonas aeruginosa azurin.