ζ-potentials of planar Si plates covered with an alkyl- or a fluoroalkyl organosilane self-assembled monolayer (SAM) were measured over a pH range of 3-11 by means of an electrophoretic light scattering spectrophotometer, and were compared with those of Si plates covered with native oxide (SiO 2 ), polyethylene (PE) and polytetrafluoroethylene (PTFE). The SAMs were prepared on SiO 2 /Si substrates by a chemical vapor deposition (CVD) method in which an alkylsilane, that is, octadecyltrimethoxysilane [CH 3 (CH 2 ) 1 7 Si(OCH 3 ) 3 , i.e. ODS] or a fluoroalkylsilane, that is, (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxysilane [CF 3 (CF 2 ) 7 CH 2 CH 2 Si(OCH 3 ) 3 , i.e. FAS] was used as a precursor. The SAM-covered Si plates were found to be 35-65% less charged than the SiO 2 /Si plate. Furthermore, SAM formation reduced surface acidity. The isoelectric points (IEPs) of the ODS- and FAS-SAM surfaces observed at pH 3.5-4 were higher than that of SiO 2 /Si (~pH 2.0). These results are attributable to a reduction in the density of surface silanol (Si-OH) groups on the SiO 2 /Si substrate. Si-OH groups were consumed due to the formation of siloxane bondings with the organosilane molecules which compose the SAMs. Finally, the IEPs of the ODS- and FAS-SAM covered surfaces were found to be almost identical to those of the PE and PTFE bulk surfaces.