The electrochemistry of a few guest molecules captured in cavities of self-assembled monolayers of thiolated cyclodextrins on gold electrodes has been examined. When electroactive molecules with different molecular dimensions such as p-benzoquinone, 2-methyl-1,4-naphthoquinone, and anthraquinone were used as guest molecules, only those with molecular dimensions smaller than the cyclodextrin cavity sizes were shown to be included by both cyclic voltammetric and electrochemical quartz crystal microbalance experiments. These results lead to the possibility that the self-assembled monolayers prepared from properly shaped compounds can be used as molecular size selective sensors. The molecules captured in the cavities were detected by linear sweep voltammetry with a detection limit of about 10(-8) M.