Au electrodes are alkylated by self-assembled organic monolayers of octadecanethiol from alcohol solution. The electron tunnelling resistance of a monolayer-coated gold electrode has been investigated by ac impedance. The relation between the fractional coverage of different defects and the corresponding film thickness at these 'collapsed' sites has been deduced from electron tunnelling theory. By using the concepts of average film thickness at defect (d(a)) and average fractional coverage of defect (theta(a)), we have obtained the theta(a) similar to d(a) plot. The influence of the apparent standard rate constant on the shape of the theta(a) similar to d(a) plot has been discussed. In our experiments, Fe(CN)(6)(3-/4-) is used as a redox probe to study the theta(a) similar to d(a) plot of an octadecanethiol monolayer. The theta(a) versus d(a) plot indicates that the defects with d(a) < 6 methylene groups and theta(a) < 0.1 can increase the apparent standard rate constant from 1.9 x 10(-10) cm s(-1), which is the theoretical value calculated from electron tunnelling theory, to 2.9 x 10(-7) cm s(-1). The average thickness of the whole monolayer (ATWM), which is obtained from the theta(a) versus d(a) plot and which can indicate the blocking property of the monolayer, is 11 methylene groups.