In　situ　monitoring　of　electrocatalytic　processes　at　solid-liquid　interfaces　is　essential　for　the　fundamental　understanding　of　reaction　mechanisms,　yet　quite　challenging.　Herein,　Pt-on-Au　nanocatalysts　with　a　Au-core　Pt-satellite　superstructure　have　been　fabricated.　In　such　Pt-on-Au　nanocatalysts,　the　Au　cores　can　greatly　amplify　the　Raman　signals　of　the　species　adsorbed　on　Pt,　allowing　the　in　situ　surface-enhanced　Raman　spectroscopy　(SERS)　study　of　the　electrocatalytic　reactions　on　Pt.　Using　the　combination　of　an　electrochemical　method　and　in　situ　SERS,　size　effects　of　Pt　on　the　catalytic　performance　of　the　core-satellite　nanocomposites　towards　CO　and　methanol　electrooxidation　are　revealed.　It　is　found　that　such　Pt-on-Au　nanocomposites　show　improved　activity　and　long-term　stability　for　the　electrooxidation　of　CO　and　methanol　with　a　decrease　in　the　Pt　size.　This　work　demonstrates　an　effective　strategy　to　achieve　the　in　situ　monitoring　of　electrocatalytic　processes　and　to　simultaneously　boost　their　catalytic　performance　towards　electrooxidation.