The kinetics and thermodynamics of the urea-induced unfolding of ¯avodoxin and apo¯avodoxin fromDesulfo-vibrio vulgariswere investigated by measuring changes in ¯avin and protein ¯uorescence. The reaction of urea with ¯avodoxin is up to 5000 times slower than the reaction with the apoprotein ( s )1 in 3 Murea in 25 mMsodium phosphate at 25°C), and it results in the dissociation of FMN.