MAP kinase modules carry out two double phosphorylation reactions, first on two serine/threonine residues, then on a Tyr residue and a Ser/Thr residue. We tested whether the reactions are sequential or random in the MAPK module comprised of the MAP3K TAO2 (or ASK1), the MAP2K MEK6, and the MAPK p38a and found that both double phosphorylation reactions (on two Ser/Thr residues in MAP2Ks, and on a Tyr residue and a Ser/Thr residue in MAPKs) occur in a precise sequence. The sequence of reactions is interesting: two Ser/Thr kinase reactions, then Tyr kinase then Ser/Thr kinase reaction, an “Excursion” into tyrosine kinase chemistry. Progress curves were fit to models for the reactions. The activities of phosphorylation intermediates were measured. We conclude that the role of the dual specificity of MAP2Ks may be to set up a precise sequence of phosphorylation reactions in MAPK modules, while starting (MAP3K) and ending (MAPK) as a Ser/Thr kinase.