Surface electrochemistry of novel monolayer-protected gold nanoparticles (MPCAuNPs) is described. Protecting ligands, (1-sulfanylundec-11-yl)tetraethylene glycol (PEG-OH) and (1-sulfanylundec-11-yl)poly(ethylene glycol)ic acid (PEG-COOH), of three different percent ratios (PEG-COOH:PEG-OH), 1:99 (MPCAuNP-COOH1%), 50:50 (MPCAuNP-COOH50%), and 99:1 (MPCAuNP-COOH99%), were studied. The electron transfer rate constants (k(cl)/s(-1)) in organic medium decreased as the concentration of the surface-exposed -COOH group in the protecting monolayer ligand is increased: MPCAuNP-COOH1% (similar to 5 s(-1)) > MPCAuNP-COOH50% (similar to 4 s(-1)) > MPCAuNP-COOH99% (similar to 0.5 s(-1)). In aqueous medium, the trend is reversed. The surface pK(a) was estimated as similar to 8.2 for the MPCAuNP-COOH1%, while both MPCAuNP-COOH50% and MPCAuNP-COOH99% showed two pK(a) values of about 5.0 and similar to 8.0. These results have been interpreted in terms of the quasi-solidity and quasi-liquidity of the terminal -OH and -COOH head groups, respectively. MPCAuNP-COOH99% excellently suppressed the voltarnmetric response of the ascorbic acid but enhanced the electrocatalytic detection of epinephrine compared to the other MPCAuNPs studied. This study reveals important factors that should be considered when designing electrode devices that employ monolayer-protected gold nanoparticles and possibly for some other redox-active metal nanoparticles.