The adsorption of CO and oxygen and CO oxidation on size-selected Pt10 clusters were studied by indirect nanoplasmonic sensing (INPS) in the pressure range of 1–100 Pa at T = 418 K. CO adsorption was reversible, inducing a blue-shift in the localised surface plasmon resonance (LSPR) response, regardless of the initial CO pressure. We observe a plateau at approximately Δλ = −0.1 nm at PCO > 2.7 Pa, indicating saturation of CO adsorption on Pt10 clusters. Oxygen induces both chemisorption and oxidation of Pt10 clusters until a regime is reached where Δλmax remains positive and constant, showing that the Pt10 clusters are completely oxidised. CO oxidation at different molar fractions is also followed by INPS. All results are discussed in relation to our previous works on 3 nm Pt nanocubes [B. Demirdjian, I. Ozerov, F. Bedu, A. Ranguis and C. R. Henry, ACS Omega, 2021, 6, 13398–13405]. The study demonstrates the suitability of INPS towards the understanding of the nature and function of matter in the largely unexplored subnanometer size regime where properties can often dramatically change when altering the particle size by a single atom.