Abstract The hydrogenation of cyclohexanone was carried out with 100% selectivity to cyclohexanol by utilizing 1–10 nm size poly(vinylpyrrolidone)-capped colloidal platinum nanocatalysts dispersed in the aqueous-phase. Particle sizes for these synthesized nanocatalysts were determined by transmission electron microscopy (TEM), and open adsorption sites on the metal nanoparticle surface were established through attenuated total reflectance infrared (ATR-IR) spectroscopic studies of chemisorbed carbon monoxide. The effect of the reaction conditions (temperature, hydrogen pressure, and cyclohexanone concentration) on turnover frequencies (TOFs) was examined to calculate apparent activation energies and reaction orders. In addition, platinum nanoparticle size and PVP molecular weight were varied to investigate effects of nanocatalyst properties on catalytic activity and reaction selectivity. In situ ATR-IR spectroscopy was used to verify adsorbed cyclohexanone as being present on the nanocatalyst surface during the reaction. Combining kinetic parameters with identification of adsorbed species is essential for developing a molecular-level mechanism for the reaction, thereby allowing factors controlling activity and selectivity in these aqueous-phase hydrogenations to be elucidated. Taken together, these results permit the ingredients essential for a highly active and selective catalyst to be discerned.