A chemical and physical characterization of particulate emitted in undiluted exhaust of single cylinder direct injection (D.I.) diesel engine was made by an optical technique.On-line scattering and extinction measurements in the spectral range from 200 to 500nm were carried out in the exhaust ofthe engine operating under steady-state conditions. These measurements provided a useful tool for the comprehension of chemical and physical structure of the particulate. They allowed the evaluation in real time of the size, the concentration and also the optical properties. Preliminary results of size and mass concentration of particulate are presented.A good agreement was observed comparing the results with those obtained by gravimetric measurements, TEM and X-ray diffraction.HIGH EFFICENCY OF DIESEL ENGINES and their ability to burn heavy fuels make them ofgreat interest in the transportation field. On the other hand, present exhaust emission levels produced by this type ofengine will not satisfy stringent international regulations on environ- menta protection.The studies of the chemical and physical structure of diesel particulate in the exhaust of the engine help manufacturers both to optimize the efficiency of the combustion process and give useful information to designers of after-treatment devices.Historically, most studies on the particulate in the engine exhaust were carried out by electron microscopy and chemical analysis [1,2]*. These conventional techniques are time consuming, not real time, and not useful in transient operating conditions. Because the composition of the particulate materia depends on the collection system the data can be influenced by sampling procedures.Size distribution measurementtechniques of particulate based on electrical mobility, diffusion and inertial separation were proposed by Dolanetal.. These techniques are real time but intrusive and can change the size distribution through collisions between particles.The recent advances in non-intrusive optical diagnostic techniques have made it possible to study on-line the evolution of particulate emitted in the exhaust with good temporal and spatial resolution.In the last decade, the optical measurements carried out in the exhaust, were made by nephetometry, transmissiometryor opacimetry, generally at a single wavelength in the visible range or in the infrared by photoacoustic and absorption measurements, These measurements are used to determine both the mass of particulate and the amount of dry soot and SOF [4,5,6,7.]In particular, simultaneous light scattering and extinction measurements at a single wavelength in the visible have provided a fast method to determine the dimension and concentration of soot particles i n flame .The purpose of the present study is to demonstrate the feasibility and flexibility of a new optical technique applied in the undiluted exhaustof a D.I. diesel engine. The technique consists of the use of simultaneous spectral measurements of scattering and extinction coefficients from the near ultraviolet to visible. These measurements allow the on-line simultaneous evaluation of the size, concentration and particulate mass as well as estimating the optical properties of the particulate. In particular, optical measurements in the near ultraviolet region are particularly promising because both graphite and large aro- maticmolecules exhibit absorption bands between 200 and 350nm [9,10].