Following a request from the European Commission, the Panel on Additives and Products or Substances used in Animal Feed (FEEDAP) was asked to deliver a scientific opinion on hexamethylene tetramine (HMTA) for pigs, poultry, bovines, sheep, goats, rabbits and horses. In 2015, the FEEDAP Panel delivered an opinion on the safety and efficacy of HMTA. In that opinion, the Panel noted that the residues of HMTA in the silage amounted to about 5% of the inclusion level of 600 mg HMTA/kg fresh ensiling material. Since the HMTA residues in silage were analysed as formaldehyde, it was not possible to quantify the contribution of formaldehyde and HMTA to the residues. In the absence of a qualitative and quantitative characterisation of the residues, the Panel could not conclude on the safety of HMTA for the target species. Based on the results of a new study in which HMTA was measured in silage, the concentration of HMTA dropped below the limit of quantification of 6 mg HMTA/kg silage, which corresponds to ~ 1% of the HMTA inclusion level in the fresh ensiling material. However, the formaldehyde concentration was not analysed in the silage. In the absence of a qualitative and quantitative characterisation of the residues, the Panel cannot conclude on the safety of HMTA for the target species.