Due to its structure, hexamethylenetetramine (HMT) is a slowly biodegradable substance, hydrolysing in acid environment to ammonium and formaldehyde. Formaldehyde is toxic under normal conditions for biological treatment processes. Following an adaptation, a formaldehyde concentration of 1000 mg-l^-1 may become biodegradable both in aerobic and anaerobic conditions. Acidic conditions for hydrolysis may be created chemically or by using natural biological treatment processes. The process of nitrification may be used in aerobic treatment, when H⁺ ions are created and pH drops below 5. For anaerobic treatment, the process of acidification may be used, when volatile fatty acids are formed and pH of the environment is decreasing. This work examines HMT degradation in an anaerobic baffled reactor (ABR). Spatial segregation of individual anaerobic degradation phases was used in the ABR, when hydrolysis and acidification in the first reactor compartments are accompanied by low pH values in these compartments. ABR performance was compared with UASB (upflow anaerobic sludge bed) reactor performance working under the same conditions. The removal efficiency of 95.8% HMT in ABR reactor and of 85.4% in UASB reactor, respectively, was measured at the organic loading 4 kg·m^-3·d^-1 and retention time 84 hours. 98% overall COD (chemical oxygen demand) removal was measured in ABR reactor and 95.6% in UASB reactor. The ABR proved to be the appropriate reactor for HMT removal. Better results of HMT removal in the ABR reactor in comparison with the UASB reactor were proved.