The　limited　fossil　fuel　reserves　and　severe　environmental　pollution　have　pushed　studies　on　improving　the　engine　performance.　This　paper　investigated　the　effect　of　hydrogen-oxygen　blends　(hydroxygen)　addition　on　the　performance　of　a　spark-ignited　(SI)　gasoline　engine.　The　test　was　performed　on　a　modified　SI　engine　equipped　with　a　hydrogen　and　oxygen　injection　system.　A　hybrid　electronic　control　unit　was　adopted　to　govern　the　opening　and　closing　of　hydrogen,　oxygen　and　gasoline　injectors.　The　standard　hydroxygen　with　a　fixed　hydrogen-to-oxygen　mole　fraction　of　2:1　was　applied　in　the　experiments.　Three　standard　hydroxygen　volume　fractions　in　the　total　intake　gas　of　0%,　2%　and　4%　were　adopted.　For　a　given　hydroxygen　blending　level,　the　gasoline　injection　duration　was　adjusted　to　enable　the　excess　air　ratio　of　the　fuel-air　mixtures　to　increase　from　1.00　to　the　engine　lean　burn　limit.　Besides,　to　compare　the　effects　of　hydroxygen　and　hydrogen　additions　on　the　performance　of　a　gasoline　engine,　a　hydrogen-enriched　gasoline　engine　was　also　run　at　the　same　testing　conditions.　The　test　results　showed　that　the　hydroxygen-blended　gasoline　engine　produced　higher　thermal　efficiency　and　brake　mean　effective　pressure　than　both　of　the　original　and　hydrogen-blended　gasoline　engines　at　lean　conditions.　The　engine　cyclic　variation　was　eased　and　the　engine　lean　burn　limit　was　extended　after　the　standard　hydroxygen　addition.　The　standard　hydroxygen　enrichment　contributed　to　the　decreased　HC　and　CO　emissions.　CO　from　the　standard　hydroxygen-enriched　gasoline　engine　is　also　lower　than　that　from　the　hydrogen-enriched　gasoline　engine.　But　NOx　emissions　were　increased　after　the　hydroxygen　addition.　Copyright　(C)　2011,　Hydrogen　Energy　Publications,　LLC.　Published　by　Elsevier　Ltd.　All　rights