Boiler deposits collected from an Australian power station were characterised and the observed deposit properties correlated with combustion residues obtained from drop tube furnace experiments using two of the power station coals noted for deposition problems. The dominant growth mechanism for deposits formed from the burner region to the screen wall was found to be inertial impaction of sticky particles. A measured systematic increase in the iron content for the deposits through the boiler was found to be due to the increased iron content required for the primarily alumino-silicate ash particles to be sticky as temperature also decreases. For the economiser region, deposit growth was found to be due to fine dry alumino-silicate and iron oxide residues deposited via the thermophoresis mechanism. It was concluded that un-associated siderite residues only contribute to deposition in the radiant section of the boiler. Deposition in the convective section was found to be due to iron alumino-silicate residues formed from included siderite associated with clays, with an increasing level of iron required in the glass with decreases in the prevailing gas temperature. This trend indicates that the included siderite grains in coal are the primary cause of slagging.