Ground-level　ozone　contamination　has　been　globally　an　urban　air　quality　issue,particu-larly　for　China,which　has　recently　made　significant　progress　in　purifying　its　sky.Unregu-lated　exhaust　emissions　from　motor　vehicles,predominantly　carbonyls　and　volatile　organic　compounds(VOCs),are　among　the　leading　contributors　to　ozone　formation.In　this　chassis-level　study,the　unregulated　emissions　from　five　China-5　certified　gasoline/CNG　bi-fuel　taxis,along　with　their　ozone　forming　potential(OFP),were　evaluated.It　is　found　that　carbonyls　and　VOCs　were　mainly　emitted　during　the　starting　phase　no　matter　the　engine　was　cold　or　hot.Compared　to　gasoline,CNG　fueling　reduced　VOCs　emissions　on　a　large　scale,es-pecially　in　the　starting　phase,but　had　elevated　carbonyls.On　a　fleet　average,CNG　fueling　derived　15％and　46％less　OFP　than　gasoline　in　cold-and　hot-start　tests,respectively.VOCs　contributed　to　over　90％of　the　total　OFP　of　the　exhaust.In　terms　of　alleviating　ground-level　ozone　contamination,CNG　is　a　feasible　alternative　to　gasoline　on　light-duty　vehicles.