The effects of intake air humidity on the performance of a turbo-charged 4-cylinder diesel engine have been investigated. The relative humidity of the intake charge was varied from 31 to 80% at a fixed ambient air temperature of 26°C. The intake humidity was controlled to within ±1% of the desired value by using a steam generator-equipped intake-air conditioning system. The tests were conducted at 3 load points (4.1, 9.1 and 15 bar BMEP) at engine speeds of 1500, 2500 and 3500 RPM without exhaust gas recirculation. The results indicate that increasing the intake air moisture leads to a reduction of 3∼14% in the NOX emissions for the tested conditions. The smoke was found to increase with speed but no significant increase in the smoke values was observed with the increased humidity. The CO and HC emissions were found to be largely insensitive to the humidity levels and were otherwise extremely low. The emissions have been analyzed on both the volumetric (ppm) and brake-specific basis to provide an insight into the effect of humidity on the quantitative results.