When　an　air-source　CO2　heat　pump　water　heater　operates　at　low　ambient　temperatures　in　cold　regions　in　winter,　frost　can　form　on　the　coil　surface　of　its　outdoor　evaporator.　The　frost　substantially　affects　the　operating　performance　and　energy　efficiency　of　CO2　heat　pump　water　heaters　and　hence　periodic　defrosting　is　essential.　In　this　paper,　defrosting　characteristics　of　an　air-source　CO2　heat　pump　water　heater　using　the　hot　gas　bypass　defrosting　method　is　experimentally　studied　at　different　ambient　conditions.　An　experimental　setup　is　developed　for　this　purpose　and　experimental　procedures　are　detailed.　Thereafter,　the　pressure　and　temperature　in　the　outdoor　evaporator,　at　the　compressor　and　gas　cooler　outlets　are　evaluated　during　the　defrosting　period.　An　energy　analysis　is　then　performed　of　different　system　components　during　the　defrosting　process.　Results　indicate　that　35%　of　the　supplied　energy　is　used　for　melting　the　frost,　and　7.6%　is　used　to　heat　the　evaporator　tubes　and　fins.　About　57.4%　of　the　supplied　energy　is　consumed　to　increase　the　internal　energy　of　the　gas　cooler.　The　typical　efficiency　of　the　hot　gas　bypass　defrosting　method　applied　in　the　CO2　heat　pump　water　heater　ranges　from　30　to　40%.　It　increases　with　increasing　dry　bulb　temperature,　and　decreasing　relative　humidity.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.