Colloidal Au nanoparticles have been functionalized with mercaptopropane sulfonate (MPS) and mercaptoethylamine (MEA) short alkanethiols in aqueous solutions. Adsorption kinetics of these charged thiols onto gold nanoparticles has been followed by the decrease in the surface plasmon band at 530 nm due to the red shift caused by particle-to-particle aggregation. Titration curves resulted from plotting the 530 and 800 nm-absorbance versus the added amount of thiol in solution, and yielded the mercaptane coverage on Au. While the adsorption of positively charged MEA on negatively charged Au colloid is very rapid, slower and more complex adsorption kinetics have been found for the negatively charged MPS.