The electrochemical behavior of folic acid at the Keggin-type phosphomolybdate (PMo12) doped polypyrrole (PPy) film modified glassy carbon electrode (PMo12-PPy/GCE) was studied. PMo12 doped PPy modified electrodes were achieved during the electrochemical preparation of the polymer films in aqueous solution. The redox behavior of the modified electrode was described by cyclic voltammetry. The electrochemical behavior of folic acid at PMo12-PPy/GCE was studied by 0.5 order differential voltammetry. Numerous factors affecting the reduction peak currents of folic acid at PMo12-PPy/GCE were optimized to maximize the sensitivity. The results showed that folic acid had high inhibitory activity toward the reduction of modified electrode in 0.01 M H2SO4. The reduction peak currents were directly proportional to the concentration of folic acid from 1.0 x 10(-8) to 1 x 10(-7) M with correlation coefficient of 0.9976, a detection limit of 1.0 x 10(-10) M of folic acid was estimated. From the inhibitory effect of folic acid on PMo12-PPy/GCE, the apparent formation constant of folic acid with the modified film was estimated. This modified electrode showed excellent sensitivity and stability for the determination of folic acid. The response mechanism of folic acid at PMO12-PPy/GCE was discussed in detail. (c) 2006 Elsevier Ltd. All rights reserved.