Tapping mode atomic-force microscopy measurements were performed for patterned self-assembled monolayers (SAMs) of -S(CH2)(15)CH3 and -S(CH2)(15)COOH groups on a polycrystalline Au substrate. Height and phase images of these SAMs were obtained as a function of the driving amplitude A(0) and the set-point amplitude A(sp). Factors influencing the contrasts of these images are discussed in terms of the simple approximation that the essential consequence of bringing the tip closer to the sample surface is to change the force constant of the cantilever.