In this study the reduction of pollutant emissions from city buses converted to dual fuel operation was investigated. Exhaust emission and fuel consumption maps were obtained under laboratory conditions for an engine converted to CNG diesel fuel operation. These values are then used in the simulation model to predict the total exhaust emission and fuel consumption on a driving cycle evaluated from actual recordings. Calculations showed a significant decrease in PM emissions as expected, while with total CO emissions minor changes have been observed. For dual fuel operation NOx emissions were kept at the same level as in pure diesel operation with retarded pilot injection. Fuel cost calculations showed a decrease up to 30 % with current prices of diesel fuel and CNG.