Robust and uniform n-alkyl monolayers were formed on silicon from the reaction of Grignard regents (n-CnH2n+1MgBr, n = 2, 6, 10, and 15) with hydrogen-terminated Si(111). The capacitive properties of these organic thin films on silicon in contact with aqueous electrolytes were evaluated by electrochemical impedance measurements. In particular, the reciprocal capacitance of the organic thin film modified silicon/aqueous electrolyte interfaces is proportional to the film thickness, which is tunable by simply varying the alkyl chain length. The derived dielectric constant of these organic thin films From the best fit of the reciprocal capacitance vs ellipsometric film thickness plot is epsilon = 3.3 +/- 0.6.