The self-assembling technique was employed for modification of a citrate-reduced silver hydrosol with mercaptoethanol (ME). The results of investigation of the structure and properties of both the self-assembled monolayers (SAMs) of ME and the modified hydrosol as a substrate for surface-enhanced Raman scattering (SERS) spectroscopy are presented. ME chemisorbs on the hydrosol by rupture of the S-H bond with concomitant bonding of the sulfur headgroup to Ag. Chemisorption is accompanied by gauche-trans isomerization of ME. The isomerization occurs cooperatively at submonolayer coverage due to interactions of neighboring adsorbed molecules rather than due to the chemisorption itself. The chemisorption of ME takes place on a less than 5 min time scale and leads to formation of a SAM with definite density and gauche-trans equilibrium, the parameters of which depend on the bulk concentration of ME but not on the time of incubation. It was revealed that densely packed and highly ordered (97% of trans-isomers) SAM is formed at an ME concentration higher than 250 mu M. ME induces concentration-dependent aggregation of the hydrosol and makes it SERS active. After 30 min of incubation with 250 mu M ME the modified hydrosol has the highest Raman enhancement only slightly altered during the next 40-50 min. Preactivation of the hydrosol surface with ClO4-, NO3-, Cl-, Br-, and I- anions does not-improve SAM formation or Raman enhancement. Using beta-carotene as a probe molecule it was well demonstrated that the structured surface of the modified hydrosol provides favorable conditions for increasing the number of adsorption sites and increasing the adsorption constant for an analyte. It also protects an analyte from direct contact with the metal surface, thereby preventing distortion of the electronic system of adsorbed molecule. These advantages make the modified hydrosol a useful tool for SERS probing of chromophores with intense electronic transitions in the visible region.