Polymer solubility modulation, typically done through solvent changes, have been the main post-polymerization method of producing polymer nanoparticles. Nanoparticles produced in this way have been used in a variety of applications, ranging from drug delivery to pollutant remediation. Herein, we report a method of preparing polymer nanoparticles through photoinduced decarbonylation of polymers bearing a bridged ketone units. Selective excitation of the n to π* absorption band of the polymer leads to quantitative decarbonylation from the polymer, resulting in a significant solubility change which could be harnessed for the preparation of well-defined polymer nanoparticles. The process is rapid and could be tracked through monitoring changes in the absorption spectra of the reaction mixture. Use of light to solubility changes in a polymer constitutes a non-invasive method of nanoparticle fabrication which could further allow for the spatiotemporal control over solubility modulation.