This article describes an experimental investigation on ash deposit formation during the co-firing of coal with agricultural residues (wheat straw, olive stones and peach stones), whose main objective was to relate the deposition rate, morphology and composition of the deposits with the type of biomass used in the co-combustion process. The study was conducted in a large-scale laboratory furnace for relatively high co-firing ratios (0.4, energy basis). For comparison purposes, tests have also been performed for pure coal firing and co-firing of coal with pine branches. During the tests, the deposits were collected with the aid of an air-cooled stainless steel deposition probe, whose surface temperature was maintained at around 500 degrees C, and of an uncooled ceramic probe with a surface temperature of about 850 degrees C. Both probes were placed in a region inside the furnace in which the mean gas temperature was about 1000 degrees C, being the deposits collected after 2 h of exposure. A number of deposit samples were subsequently analyzed on a scanning electron microscope equipped with an energy dispersive X-ray detector. The results reveal that, in contrast with the co-firing of coal with pine branches, co-firing of coal with wheat straw and olive stones increase the ash deposition rate, as compared to pure coal firing. In addition, deposits resulting from the use of wheat straw and olive stones as secondary fuels present higher degree of adherence to surfaces. This is due to the high concentration of alkali metals present in the deposits composition, leading to compounds of low melting points. In particular, these two biomass fuels originated deposits with relatively high levels of K and S, which facilitate their sintering degree. The use of peach stones in the co-firing process led to a deposition rate similar to that of pure coal firing. However, the deposits present a relatively higher degree of adherence to the surfaces, which is probably due to the presence of higher levels of Na in its composition. (C) 2013 Elsevier Ltd. All rights reserved.