DiFMUP is a fluorinated 4-methylumbelliferyl phosphate (MUP) derivative that has a lower pKa than that of MUP, making DiFMUP an excellent substrate for continuously assaying acid phosphatases at low pH. The reaction product of DiFMUP has excitation/emission maxima of ∼358/450 nm. DiFMUP is also available packaged in ten vials of 10 mg each (D-22065).