The decomposition of methyl parathion (an organothiophosphate pesticide) sorbed on partially hydrated kaolin and montmorillonite clays (in Ca2+, Cu2+, Zn2+, and Al3+ forms) at high concentration (typically 1-10 wt %) has been examined by nuclear magnetic resonance (NMR), using solid-state 31P NMR (based on magic-angle spinning and cross polarization or direct polarization) and liquid-state 31P NMR of DMSO and acetone extracts. The results indicate that methyl parathion is initially physisorbed, appearing by solid-state 31P NMR to exhibit substantial molecular-level motion. The signals due to unreacted methyl parathion diminish and are replaced by new 31P NMR peaks resulting from hydrolysis, isomerization, and oxidation reactions over periods ranging from hours to years. 31P NMR characteristics indicate that these decomposition products are much more tightly bound to the clay than is methyl parathion. Methyl parathion decomposition is most effectively catalyzed by partially hydrated Cu(II)- and Al-montmorillonites (but with different product distributions); Ca-montmorillonite and kaolin were least effective.