Use of ethanol as gasoline replacement can contribute to the reduction of nitrogen oxide (NOx) and carbon oxide (CO) emissions. Depending on ethanol production, significant reduction of greenhouse-gas emis- sions is possible. Concentration of certain species, such as unburned ethanol and acetaldehyde in the engine-out emis- sions are known to rise when ratio of ethanol to gasoline increases in the fuel. This research explores on hydrous ethanol fueled port-fuel injection (PFI) spark ignition (SI) engine emissions that contribute to photochemical formation of ozone, or so-called ozone precursors and the precursor of peroxyacetyl nitrates (PANs). The results are compared to engine operation on gasoline. Concentration obtained by FTIR gas analyzer, and mass-specific emissions of formal- dehyde (HCHO), acetaldehyde (MeCHO) and methane (CH4) under two engine speed, four load and two spark advance settings are analyzed and presented. Combustion phasing results based on in-cylinder pressure analysis are also included. Concentration and specific emissions of acetalde- hyde and methane were found to increase in case of hydrous ethanol, comparing to gasoline use. Difference in formalde- hyde emissions was statistically insignificant at slower engine speed. Introduction