The use of CNG in light-duty vehicles is well known and widespread in Brazil, mainly in the state of Rio de Janeiro. In theory, the application of this technology could provide greater energy efficiency and lower emissions of pollutants. However, it is known that the majority of CNG kits installed do not meet the environmental requirements established by the Brazilian legislation.This work quantifies the average emissions from light-duty vehicles converted to CNG use in the Metropolitan Area of Rio de Janeiro, where 47% of the Brazilian CNG fleet runs. For this purpose, initially a field survey of gas stations was performed in this Region, in order to define the representativeness of CNG kits in terms of technology generation and brand, as well as manufacturer, model, displacement, original fuel and manufacture year of vehicles in which such kits were installed.Based on the results of this survey, six vehicles adapted with CNG kits were selected for FTP-75 tests of emissions and fuel economy in the Vehicle Testing Laboratory of PETROBRAS Research Center (CENPES). Then it was calculated a weighted average of the emission of THC, NNHC, CH₄, CO, NOx, aldehydes and CO₂ in vehicles with CNG kits in the Metropolitan Area of Rio de Janeiro.