The electrochemistry of 3,4,9,10-perylene tetracarboxylic acid (PTCA) immobilized on graphene surface (PTCA/G) via pi-pi stacking shows four pairs of current peaks in cyclic voltammogram. These peaks are ascribed to sequential protonation and deprotonation of carboxyl groups in PTCA. The distinct differences in four peak potentials demonstrate the existence of electrostatic and hydrogen-bonding interactions between the four carboxyl groups. In addition, it is found that the kinetics for the protonation and deprotonation of PTCA/G changes from surface controlled process at low scan rate to diffusion controlled one at high scan rate. (C) 2012 Elsevier B.V. All rights reserved.