Tests were conducted on Diesel engines to determine the cause of exhaust discoloration. Results show that the discoloration occurs for different reasons under different engine conditions (e.g., fully loaded, steady-state operation, full acceleration, etc.). The paper correlates the different discolorations with their respective engine processes. It is concluded that the present Hungarian measurement standard is inadequate.Under the assumption that commercial, city traffic is comparable to personal vehicle traffic, the Europa-Test standard should be adopted whereby the thickness of exhaust effluent is continuously measured. Mechanisms employing light beams and sensors are in development and could be readily installed in the end of the exhaust pipe. Test results would be recorded along with other data such as engine rpm, temperature, etc. Tests can subsequently be done on any appropriate road rather than at test-stands. The measurements would be more characteristic of actual engine operations.