Electrocatalytic oxidation of formaldehyde was investigated with the aim of developing highly active electrodes for fuel cells. Polyaniline prepared in an ionic liquid, 1-ethylimidazolium trifluoroacetate (PAN-HEImTfa), was composed of significantly regular nano-particles with an improved electroconductivity. Compared to polyaniline prepared in H2SO4 and a bare Pt electrode, PAN-HEImTfa exhibits the highest electrocatalytic activity for formaldehyde oxidation in sulfuric acid. After cyclic voltammetry of PAN-HEImTfa in 0.5 M H2SO4 containing 1.0 M formaldehyde for 80 cycles, electrocatalytic activity of PAN-HEImTfa is still stable without an obvious decrease in the peak currents, and no detachment of the PAN-HEImTfa from the Pt matrix was observed. (C) 2005 The Electrochemical Society.