The biocide polyhexamethylene biguanide (PHMB) has been characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Previously, no method has been able to provide a detailed structural characterization of PHMB. MALDI-TOF MS was able to detect PHMB oligomers with n <= 6. Six different PHMB product types were identified, which possess combinations of amine, cyanoamine, guanidine, or cyanoguanidine end-groups. Postsource decay (PSD) fragmentation was used to confirm the correct assignment of PHMB structure for the dominant PHMB molecular ion. MALDI-TOF MS analysis of a N-15-labeled PHMB confirmed the correct assignment of PHMB molecular ions, and also indicated the existence of a polymerization-depolymerization equilibrium during melt polymerization of the polymer. (c) 2006 Wiley Periodicals, Inc.