Abstract: We report the first application of 39K solid-state NMR to the study of tecto- and phyllosilicates. Under high field (11.7 Tesla) and with the application of a spin-echo sequence, informative 39K spectra can be obtained for several compounds of interest to the geologist and the agronomist. Tectosilicates and phyllosilicates can be distinguished from the uncorrected frequency (δCG) of the observed NMR peak. A series of montmorillonites submitted to increasing numbers of wetting and drying cycles was studied in order to discriminate between mobile and “fixed” forms of K+: when the spectra are run on hydrated samples, two different signals are observed corresponding to K+ in different hydration states, and NMR data can be correlated with the amount of exchangeable K+ measured by ion exchange. Thus, it appears that NMR can provide useful information on K fixation complementary to classical chemical methods.