The flash dehydration of a ulexite mineral in an entrained flow reactor was investigated. Flash dehydration experiments were carried out in the range 400-700 degrees C with material of particle size -425+300 mu m. The weight loss at 700 degrees C dehydration temperature was measured as 22%. A thermal gravimetric method was used to obtain the activation energy and rate constants for the dehydration of ulexite. The activation energy was in the range 48.2-53 kJ mol(-1) for different particle sizes. The rate constants for different particle sizes were found to vary between 1.8 x 10(-4) and 1.35 s(-1) in the temperature range 110-530 degrees C.