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$^{63/65}$Cu- and $^{35/37}$Cl-NMR experiments were performed to investigatetriplet localization in the $S=1/2$ dimer compound NH$_4$CuCl$_3$, which showsmagnetization plateaus at one-quarter and three-quarters of the saturationmagnetization. In $^{63/65}$Cu-NMR experiments, signal from only the singlet Cusite was observed, because that from the triplet Cu site was invisible due tothe strong spin fluctuation of onsite 3$d$-spins. We found that the temperaturedependence of the shift of $^{63/65}$Cu-NMR spectra at the singlet Cu sitedeviated from that of macroscopic magnetization below T=6 K. This deviation isinterpreted as the triplet localization in this system. From the$^{35/37}$Cl-NMR experiments at the 1/4-plateau phase, we found the twodifferent temperature dependences of Cl-shift, namely the temperaturedependence of one deviates below T=6 K from that of the macroscopicmagnetization as observed in the $^{63/65}$Cu-NMR experiments, whereas theother corresponds well with that of the macroscopic magnetization in the entireexperimental temperature region. We interpreted these dependences as reflectingthe transferred hyperfine field at the Cl site located at a singlet site and ata triplet site, respectively. This result also indicates that the triplets arelocalized at low temperatures. $^{63/65}$Cu-NMR experiments performed at highmagnetic fields between the one-quarter and three-quarters magnetizationplateaus have revealed that the two differently oriented dimers in the unitcell are equally occupied by triplets, the fact of which limits the theoreticalmodel on the periodic structure of the localized triplets.

$^{63/65}$Cu- and $^{35/37}$Cl-NMR Studies of Triplet Localization in the Quantum Spin System NH$_4$CuCl$_3$