This paper deals with the development of a laboratory reactor to study ultrafine (D < 100 nm) and nano (D < 30 nm) ash formation during pulverized coal combustion and oxy-firing. The reactor consists of an atmospheric pressure flat laminar premixed flame homogeneously doped with pulverized coal particles, monodisperse in size. It is accessible to diagnostics and sampling systems and it allows investigating the early stage of particle formation in a wide range of pulverized coal combustion operative conditions, in terms of gas composition and temperature. Coal combustion in an oxygen enriched gas mixture was investigated by performing on-line high resolution differential mobility analyses (DMA) and thermophoretic samplings for atomic force microscopy (AFM) image analyses. Ultrafine particle size distribution functions in a size range extending down to 1 nm have been measured. Three types of high volatile bituminous coals have been tested. Ultrafine particles, commonly neglected at the exhaust of pulverized coal combustors, form with huge number concentration and they represent a not negligible fraction of total ashes also in volume/mass. Nano-ashes are the most abundant in number and they also significantly contribute to ultrafine particle mass concentration. This not negligible contribution slight increases with the coal chlorine content while the shape of the nano-ash size distribution function is quite unaffected by the used coal type. (C) 2010 Elsevier Ltd. All rights reserved.