Adsorption isotherms and UV-visible and Mössbauer spectroscopic data point to specific interactions between flavomononucleotide (FMN) and Fe3+-smectite. The maximum amount of FMN adsorption was 0.3 mmole/g of Fe3+-smectite giving a 1:1 molar proportion of Fe3+ and FMN. The results suggest a Fe3+-FMN complex residing at the smectite surface. Other homoionic smectites (Cu2+, Zn2+, and Ca2+) exhibited lower levels of adsorption and less apparent specific interaction.