Dehydration kinetics of sodium perborate tetrahydrate to monohydrate is investigated at four different temperatures between 314.7 and 331 K in a controlled fluidized-bed drier. During the dehydration, no thermal decomposition of sodium perborate to sodium metaborate is observed. First-order kinetic behavior in this conversion is proved and the activation energy for dehydration is calculated as 80.18 kJ/mol. Effect of temperature on the bulk density of the product is given and compared with theoretical calculations. Change in the bulk density is attributed to two different competitive effects, one is shrinking and the other is puffing. Bulk density of the product can be adjusted by controlling these two parameters. (C) 1998 Elsevier Science Ltd. All rights reserved.