Splitting　of　water　to　hydrogen　and　oxygen　on　colloidal　catalysts　is　a　promising　method　for　future　energy　and　chemistry　cycles.　The　currently　used　high-performance　oxides　containing　expensive　elements　(Ru,　Ir)　are　progressively　being　replaced　by　more　sustainable　ones,　such　as　Co3O4.　Although　the　size　of　the　nanoparticles　determines　their　catalytic　performance,　the　control　over　the　particles＇　diameter　is　often　synthetically　difficult　to　achieve.　An　additional　obstacle　is　the　presence　of　stabilizing　agent,　an　organic　molecule　that　blocks　accessible　surface-active　centers.　Herein,　we　present　how　precise　control　over　size　of　the　cobalt　oxide　nanoparticles　(Co　3O4　NPs),　their　colloidal　stability,　and　the　ligand-free　surface　affect　overall　performance　of　the　photocatalytic　oxygen　evolution.　We　accordingly　correlated　the　photochemical　results　with　the　electrochemical　studies,　concluding　that　accessibility　of　the　active　species　on　the　particles＇　surface　is　crucial　parameter　in　water　oxidation.　©　2013　American　Chemical　Society.