Understanding the formation of nanoparticles (NPs) is key to develop materials by sustainable routes. The Co4Cat(TM)process is a new synthesis of precious metal NPs in alkaline mono-alcohols well-suited to develop active nanocatalysts. The synthesis is 'facile', surfactant-free and performed under mild conditions like low temperature. The reducing properties of the solvent are here shown to strongly influence the formation of Pt NPs. Based on the in situ formation of CO adsorbed on the NP surface by solvent oxidation, a model is proposed that accounts for the different growth and stabilization mechanisms as well as re-dispersion properties of the surfactant-free NPs in different solvents. Using in situ and ex situ characterizations, it is established that in methanol, a slow nucleation with a limited NP growth is achieved. In ethanol, a fast nucleation followed by continuous and pronounced particle sintering occurs.