Abstract: This invention provides for the active site mapping of enzymes which catalyse covalent modification including, but not limited to phosphorylation, acylation, dephosphorylation in which a fixed residue (known as the catalytic residue) such as a tyrosine, serine, threonine, histidine, aspartic acid residue or any other residue containing an appropriate side chain is modified. Mapping of protein kinases is exemplified. The method of the invention has an additional level of complexity over and above that of the self-deconvoluting libraries described in W097/42216. This involves making a library of smaller libraries (referred to as subsets) where a fixed residue is moved stepwise through the sequence of amino acids or other groups (such as peptidomimetics). Using 5 subsets of libraries of peptides of 5 amino acids allows the mapping of a sequence of 9 amino acids.