Organized monolayers of octadecanethiol on gold are examined by cyclic voltammetry (CV) and ac impedance spectroscopy (ACIS) in electrolytes containing 1-electron redox couples. Pinholes in the monolayer lead to voltammetric behavior resembling that of a microarray electrode. Contour plots are given for linear scan voltammetric parameters (peak current, peak potential and half-peak potential) from the theoretical treatment of microarray electrodes. The theory of ac impedance spectroscopy is developed for a microarray electrode with the area fraction of active sites less than 0.1. The behavior of monolayer-coated electrodes conforms qualitatively to the predictions for microarray electrodes with pinhole radii of ca. 0.1-10 mum and pinhole separations of 1-100 mum, but significant deviations from the predictions are observed. The deviations are discussed in terms of the nonuniform distribution of the diameter and separation of the pinholes. ACIS data in particular suggest that the pinholes are grouped in patches.