Size-quantized CdS nanocrystals serve as photocatalysts for nitrate reduction at neutral DH under conditions that mimic illumination by sunlight with overall product quantum yields of up to 4% for similar to 20 Angstrom, amine-terminated particles. Due to the effects of quantum confinement on electron and hole redox potentials, photocatalyzed nitrate reduction rates depend strongly on the apparent particle size, and the fastest reduction rates are observed with the smallest nanocrystals which have the highest exciton energy, Using a Tafel plot and the empirical pseudopotential model to estimate electron redox potentials, the apparent electron transfer coefficient and the apparent standard rate constant are estimated at 0,23 and 4.0 x 10(-12) cm/s, respectively, for amine-terminated particles. The apparent values for these constants indicate sluggish kinetics and the probable influence of adsorption and double-layer effects on the observed reaction rate. The effect of nitrate adsorption on photoreduction rates is described well by a Langmuir-Hinschelwood expression, Nitrate reduction rates are reduced 2-fold or more on negatively charged, carboxy-terminated nanocrystals that electrostatically repel nitrate. Chloride competes with nitrate for access to particle surfaces, and reduced photoreduction rates are observed for both amine- and carboxy-terminated particles with increased NaCl concentration. The rate of photocatalyzed nitrate reduction on the amine-capped particles goes through a minimum at about pH 6.5, whereas the efficiency of nitrate reduction for the carboxy-terminated system decreases monotonically with increasing pH. in the absence of an electron donor ether than water, rapid photocorrosion is observed, therefore, formate is used as the sacrificial electron donor in this study.