Hydroxylamine reacts as an oxygen nucleophile, most likely via its ammonia oxide tautomer, towards both phosphate di- and triesters of 2-hydroxypyridine. But the reactions are very different. The product of the two-step reaction with the triester TPP is trapped by the NH2OH present in solution to generate diimide, identified from its expected disproportionation and trapping products. The reaction with H3N+-O- shows general base catalysis, which calculations show is involved in the breakdown of the phosphorane addition-intermediate of a two-step reaction. The reactivity of the diester anion DPP- is controlled by its more basic pyridyl N. Hydroxylamine reacts preferentially with the substrate zwitterion DPP ± to displace first one then a second 2-pyridone, in concerted SN2(P) reactions, forming O-phosphorylated products which are readily hydrolysed to inorganic phosphate. The suggested mechanisms are tested and supported by extensive theoretical calculations.