Corrosion inhibition mechanism of aluminium in H2SO4 by polyacrylic acid (PAA) in the absence and presence of iodide ions was investigated using in-situ atomic force microscopy (AFM). Results indicate that PAA inhibited the corrosion of aluminium by adsorption onto the aluminium surface. Inhibition efficiency of PAA was found to enhance on addition of iodide ion which could be ascribed to synergistic effect. In-situ AFM morphology of the surface film suggests that PAA in the presence of iodide ion was adsorbed on aluminium surface after taking a more ordered arrangement, thus providing a more uniform coverage at potentials below and above potential of zero charge.