Adsorption of a series of primary substituted hydrocarbons (RX; C18H37PO(OH)(2) (ODPA), C17H35COOH, C18H37OH, C18H37NH2, and C18H37SH) onto solid gallium substrates with and without UV/ozone treatment was studied using contact angle goniometry, spectroscopic ellipsometry, and cyclic voltammetry (CV). UV/ozone treatment offered a hydrophilic surface (water contact angle (theta(water)) less than 10 degrees), reflecting the formation of a surface oxide layer with the maximum thickness of ca. 1 nm and possibly the removal of surface contaminants. Upon immersion in a toluene solution of a Rx, theta(water) increased due to adsorption of the RX onto gallium substrates. In particular, UV/ozone-treated gallium substrates (UV-Ga) immersed in an ODPA solution exhibited theta(water) close to 105 degrees. The ellipsometric thickness of the adsorbed ODPA layer was ca. 2.4 run, and CV data measured in an acetonitrile solution showed significant inhibition of redox reaction on the substrate surface. These results indicate the formation of a densely packed ODPA monolayer on UV-Ga. The coverage of a C17H35COOH layer adsorbed onto UV-Ga was lower, as shown by smaller theta(water) (ca. 99 degrees), smaller ellipsometric thickness (ca. 1.3 nrn), and smaller electrode reaction inhibition. Adsorption of the other RX onto UV-Ga was weaker, as indicated by smaller theta(water) (82 degrees-92 degrees). ODPA did not strongly adsorb onto UV-untreated gallium substrates, suggesting that the ODPA adsorption mainly originates from hydrogen bond interaction of a phosphonate group with surface oxide. These results will provide a means for controlling the surface properties of oxide-coated gallium that play an essential role in monolayer conductivity measurements and electroanalytical applications.