The Fe oxide catalysts were suspended in an aliph. olein-contg. solvent (b.p. 180-220 degrees C) at 90 degrees C for 6 h. The stable suspensions showed a particle size distribution of about 50 nm. Heating oils modified with addn. of the catalysts were combusted. The CO, NOx, CHx and particulate matter concns. in exhaust gases were measured. Lower emission was obsd. for the Fe(III) oxide-contg. catalyst.