Screening of a modest library of 352 fragments afforded hits for two of the three proteins assessed, which were found to interact with functionally relevant pockets. An advantage of screening with reactive fragments over classical reversible fragments, is the facile identification of the site(s) of covalent modification using tandem MS. Hits identified here were found to modify tyrosine and histidine residues. The observation of histidine modification under the denaturing mass spectrometry workflows was perhaps surprising, since previous reports have suggested that histidine adducts of SFs can be unstable. The multiple examples of histidine modification provide evidence that this is not always the case and that SFs are suitable electrophiles for targeting histidine residues. Kinetic analyses of the CAII hits provided a measure of covalent modification efficiencies and parameters kinact and KI. Interestingly, the CAII original hits exhibited good non-covalent affinity (KI) but slow rates of covalent modification (kinact). This suggested either low nucleophilicity of the histidine residue or sub-optimal geometry for covalent modification. A HTC-D2B screen of analogues close to the original hits identified fragments that gave fast rates of modification (>70% covalent modification in <1 h), highlighting the role of electrophile position in the design of covalent inhibitors, and the utility of the HTC-D2B approach as a means to optimise both kinact as well as KI.