The deposition of sludge formed in engines can be prevented quite effectively by dispersants. However, these are incorporated into the sludge at higher levels than necessary for the dispersant action. A better knowledge of the creation and nature of the sludge could help in devising ways to prevent the formation or deposition more efficiently.Chemical and physical methods have been used to characterize the sludge and its growth. The chemical method involves a slow thermal degradation and the determination of the nature and the rate of evolution of degradation products. These can provide information on oil, fuel, additive or engine conditions. We could also optimize a laboratory method for synthesizing an artificial sludge resembling the engine material more closely than possible hitherto. The physical state of the sludge, i.e. the growth in size and number of the particles, has been investigated by a special centrifugation technique. Single drops of oil suffice to reveal the sludge situation during engine tests. The effects of ashless dispersants e.g. on particle size are better defined.