Electrodes modified with polypyrrole-flavin mononucleotide composite film (PPy-FMN) have been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and W-visible spectro-electrochemistry with optically transparent electrodes (OTE). Flavin molecules, electrostatically trapped in the composite film, act as dopant anions and can be reduced chemically by reduced PPy0 or electrochemically with a conducting PPy backbone connecting them to the underlying electrode, Both the electrochemistry and the electronic spectra of FMN entrapped in the PPy film markedly differ from those of FMN in aqueous solutions. These differences arise from the interactions of flavin molecules in a rigid matrix and the possibility of both chemical reduction of FMN by reduced PPy0 and flavin electro-reduction in the host polymer matrix.