The increasing urbanization level of many countries around the globe has led to a rapid increase of mobility demand in cities. Although public transport may play an important role, there are still many people relying on private vehicles, and, especially in urban areas, motorcycles and scooters can combine handling and flexibility with lower cost of operation compared to passenger cars. However, in spite of their lower fuel demand, they might significantly contribute to air pollution, lagging behind cars in terms of emission performances.The aim of this paper is to provide the scientific community with the results of an exploratory test campaign on four different motorcycles, converted from gasoline to CNG by means of an after-market conversion kit. A fifth motorcycle, similarly converted from gasoline to LPG, was also tested. These vehicles are powered by 4-strokes engines with a displacement ranging from 50 to 250 cm3 and a power ranging from 3.0 to 16.5 kW. They have been tested over the relevant phases of the ECE-47, ECE-40 and WMTC driving cycles, with both gasoline and gaseous fuel. Gaseous emissions results show the dependence of HCs, CO, CO2, NOx and CH4 on the engine displacement, fuel, injection and catalyst technology.Additionally the energy efficiency and the environmental impact of the different tested configurations have been analyzed in terms of equivalent energy consumption and GWP. The results suggest that the CNG fuel with the appropriate injection technology might be a viable solution to combine low cost of operation with low gaseous emissions and global warming impact. The tests underline the need of an optimized setup of the conversion kit to avoid unwanted increase of the emissions.