The loss of alkali metal elements from high-temperature molten biomass slag (wood and rice straw) can be related to the extent of polymerization of the melt. If the alkali metals occur as network-modifying and charge-balancing cations in highly depolymerized melts, such as wood slag, they are easily evaporated during prolonged heating and subsequently deposited on heat exchangers. If the melt is highly polymerized, such as rice straw slag, where the alkali metals occur as network-modifying cations, they are strongly retained in the polymerized network. These differences can be related to the availability of large-sized and low-density charged melt positions. Rice straw ash melt is dominated by a relatively open polymerized network that will easily accommodate the large Na+ and K+ ions. Wood ash melt is highly depolymerized and does not easily accommodate the large K+ ion and only to a certain extent the Na+ ion, but will accommodate the smaller and more highly charged Ca2+ ion. Therefore, the alkali metals in wood slag melt are strongly partitioned into the vapor phase, with K preferentially lost relative to Na from the liquid phase. It is a consequence of this study that the use of straw fuels, compared to wood fuels, may significantly reduce the alkali loss from high temperature molten slag. It is tentatively estimated that about 70% of potassium in rice ash may be retained in the slag. This is in contrast to wood ash where all potassium is lost to the combustion gas with prolonged heating. However, the highly polymerized nature of rice and wheat straw melts and their low melting points render these straws less attractive as fuels for many biomass-fueled power plants.