The　gasoline　engines　always　encounter　the　deteriorated　thermal　efficiency　and　increased　toxic　emissions　at　part　load　conditions.　This　paper　investigated　the　effect　of　hydrogen/oxygen　blends　(hydroxygen)　addition　on　the　performance　of　a　gasoline　engine　at　different　hydrogen　volume　fractions　in　the　hydroxygen.　The　experiment　was　conducted　on　a　1.6　L　gasoline　engine　equipped　with　a　hydrogen　and　oxygen　port　injection　system.　A　hybrid　electronic　control　unit　was　adopted　to　control　the　spark　timing　and　the　injection　timings　and　durations　of　hydrogen,　oxygen　and　gasoline.　The　test　was　performed　at　a　typical　city　driving　speed　of　1400　rpm,　a　manifolds　absolute　pressure　of　61.5　kPa　and　two　excess　oxygen　ratios　of　1.00　and　1.20.　The　overall　volume　fraction　of　the　hydroxygen　in　the　total　intake　gas　was　fixed　at　3%.　The　hydrogen　volume　fraction　in　the　hydroxygen　was　raised　from　0%　to　100%　by　changing　the　injection　durations　of　hydrogen　and　oxygen.　The　test　results　demonstrated　that　the　engine　thermal　efficiency　was　obviously　increased　with　the　increase　of　hydrogen　volume　fraction　in　the　hydroxygen.　The　fuel　energy　flow　rate　of　the　3%　hydroxygen-blended　gasoline　engine　was　lower　than　that　of　the　original　engine　when　the　hydrogen　volume　fraction　in　the　hydroxygen　exceeded　70%.　Both　the　flame　development　and　propagation　periods　were　shortened　after　the　hydroxygen　addition.　HC,　CO　and　NOx　emissions　were　decreased　with　the　increase　of　hydrogen　volume　fraction　in　the　hydroxygen.　But　NOx　emissions　of　the　hydroxygen-blended　engine　were　higher　than　those　of　the　original　engine　for　all　hydrogen　volume　fractions　in　the　hydroxygen.　Moreover,　at　an　excess　oxygen　ratio　of　1.00,　CO　from　the　3%　hydroxygen-blended　gasoline　engine　was　also　higher　than　that　from　the　original　engine.　The　reduced　particulate　emissions　can　be　obtained　only　at　relatively　high　hydrogen　volume　fractions　in　the　hydroxygen.　Copyright　(C)　2012,　Hydrogen　Energy　Publications,　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.