This document specifies a method for the determination of methylen-urea (MU) oligomers in inorganic fertilizers using high-performance liquid chromatography (HPLC). The method is applicable to all fertilizers which do not contain interfering organic compounds. NOTE By the condensation of urea and formaldehyde, several oligomers are formed, such as methylen diurea (MDU), dimethylen-triurea (DMTU), trimethylen-tetraurea (TMTU) and higher oligomers. The three molecules named here are the most soluble in water, while the higher compounds are insoluble in hot water, but their nitrogen is available for plants by microbiological decomposition. Also, urea is always a companion of MU-oligomers.