A humid air motor system (HAM) for NO\dx reduction has been connected to an eleven liters diesel engine. Earlier studies have demonstrated that the system has capacity to lower NO\dx emissions from diesel engines. The present study is directed to investigate the influence of the system on the emissions of particles and aldehydes, to monitor essential engine parameters, water consumption, and to verify the NO\dx reducing ability. The system has been tested under the various speed and load conditions stated in the 13-mode ECE R- 49 test procedure. Additional tests have been carried out for sampling and measurements of particles. The results showed that the particle number concentration usually increased when HAM was coupled to the engine. The increase in particle number concentration, observed in five out of six running modes, varied between 46% and 148%. There was no observed trend indicating a shift in mean particle diameter when using HAM. Engine performance was almost unaffected, while the HAM system caused a large reduction of the NO\dx emissions. Even without optimization of the experimental conditions the average reduction of NO\dx during the different modes of ECE R-49 was over 51%. The reduction was directly related to the humidity of the inlet air and a further reduction can be anticipated with higher humidity. The influence of the system on the emissions of hydrocarbons (HC) was negligible while a moderate increase in the emission of carbon monoxide (CO) was noticed. No confident relationship between air humidity and the observed effects (i.e., on HC and CO) could be detected. Samples were also taken for acetaldehyde and formaldehyde. The results are indicating a reduction of aldehydes in the range 78% to 100%, when using HAM. Unfortunately these large reductions may be false, since it cannot be excluded that they are a result of a combination of high air humidity and the sampling technique used.