Gold electrodes modified with monolayers with ionic end groups of different acidity and structure have been evaluated for:he detection of dopamine. It has been demonstrated that the selectivity of monolayers depends not only on ionic charge but also on the nature of the ionic end group and defects in the monolayer. Catalytic effects due to the electrocatalytic oxidation of ascorbic acid by dopamine are present at the monolayer modified gold electrode. The angle resolved XPS (X-ray photoelectron spectroscopy) experiments demonstrate that the sulfonate group do not compete with thiol on binding to the gold electrode. The capacitance measurements demonstrate that the capacitance depends on the length, end group type and defects present in monolayers.