The secondary pollutants formation in the atmosphere, such as ozone (O3), comes from the reactions between volatile organic compounds (VOCs) and the photochemical oxidants in the presence of oxygen and nitrogen oxides (NOx). The understanding of VOCs reactivity emitted by light duty vehicles is very important to construct reactivity scales regarding ozone formation.In 2003, flex-fuel vehicles were released in the Brazilian market and nowadays, they represent over 50% of the total light duty vehicle fleet in the country. In 2007, new tailpipe emission limits were implemented for Non-Methane Hydrocarbons (NMHC), a group of pollutants included in VOCs. The new NMHC limit became a challenge to homologate some flex-fuel models, when fueled with ethanol. To deal with this issue, other legislation allowed the subtraction of the unburned ethanol emission from the NMHC value. In 2010, a local environmental agency published a study mentioning the need to revise this subtraction, since the unburned ethanol can also contributes to ozone formation. To better understand this issue, it was created, in 2012, a working group in Brazil, to discuss about vehicle exhaust gases reactivities to form ozone.This paper presents theoretical concepts and a calculation methodology to estimate the potential reactivity of the VOCs to form ozone. It also includes emission experimental results for different vehicles using gasoline, ethanol and Compressed Natural Gas (CNG).