The elemental losses from ashes of common biomass fuels (rice straw, wheat straw, and wood) were determined as a function of temperature from 525 8C to below 1525 8C, within the respective melting intervals. The experimental procedure was chosen to approach equilibrium conditions in an oxidizing atmosphere for the specific ash and temperature conditions. All experiments were conducted in air and used the ashes produced initially at temperatures of 525 8C as reactants. Losses during the initial ashing at 525 8C were negligible, except for a K2O loss of 26% for wood and a Cl loss of 20% for wheat straw. Potassium losses are positively correlated with temperature for all fuel ashes. The K2O loss for wood ash commences at 900–1000 8C. Carbonate is detected in the wood ashes to about 700–800 8C and thus cannot explain the retention of K2O in the ashes to 1000 8C. Other crystalline phases detected in the wood ashes (pericline and larnite) contain little or no potassium. Petrographic examinations of high temperature, wood ash products have failed to reveal potassium bearing carbonates, sulfates, or silicates. The release of potassium, thus, appears to be unrelated to the breakdown of potassium-bearing crystalline phases. The straw ashes show restricted potassium loss compared to wood ash. The potassium content declines for both straw ashes from about 750 8C. Cristobalite appears in the straw ashes at about 700–750 8C and is replaced by tridymite in the rice straw ash from about 1100 8C. Sylvite (KCl) disappears completely above 1000 8C. The Cl content starts to decline at about 700 8C, approximately at the same temperature as potassium, suggesting that the breakdown of sylvite is responsible for the losses. The K–Cl relations demonstrate that about 50% of K (atomic basis) released from breakdown of sylvite is retained in the ash. The presence of chlorine in the ash is, therefore, best attributed to the presence of sylvite. Potassium is easily accommodated in the silicate melt formed at temperatures perhaps as low as 700–800 8C from dehydration, recrystallization, and partial melting of amorphous components. Loss of potassium persists for ashes without remaining sylvite and points to the importance of release of potassium from partial melt at temperatures within the melting interval for the fuel ashes.