Rice husk can be considered as an "opportunity fuel" for energy production. However, although rice husk has long been identified as a source for energy production, only limited experience exists from rice husk firing in larger-scale combustors. Only a few units worldwide are reported to be using rice husk as their main fuel. One concern in rice husk firing is the behavior of the ash, i.e., its slagging and fouling tendency, as well as its abrasiveness. This paper presents the very characteristic properties of the rice husk ash, as measured by a variety of laboratory tests and analyses, and compares these characteristics with eucalyptus bark and rice straw, as well as with some other biomass fuels. The paper is the first in a series of two, where we report from a recently finished study on the slagging and fouling behavior of rice husk when fired alone or in combination with other fuels in a fluidized-bed boiler. In the second part of the series, we will report the results of fireside fouling measurements in a pilot-scale burning test facility and in a 157 MWth full-scale fluidized-bed boiler burning rice husk and bark in different ratios. Rice husk has been shown to produce large, almost millimeter-sized ash particles with a characteristic shape. The particles consisted almost purely of silica, with just a few percent of potassium. The melting properties for this type of ash is characterized by a very high initial melting temperature, and, consequently, no molten fly ash is expected under any fluidized-bed combustion conditions when husks are burned alone.