The electrochemical reduction processes of heteropolyanion PMo12O3-40 in protonic media were investigated by in-situ FTIR spectroelectrochemical method’ Experimental re-sults indicated that the protonational site of electroreduction product HnPMo12O3-40(n= 2, 4,6, 8) was on bridge oxygen atom- The infrared characteristic absorption peaks of Mo = 0double bound shift to higher frequence after electroreduction of heteropo1yanion PMo12O3-40.This result can be interpreted from the viewpoint of induction effect of proton.