Abstract: Solutions of 10 amino acids in water, 2M KCI and 0.5M K2S04 were analysed to determine their possible interference using the Berthelot colour reaction. The results confirmed the suspicion that amino acids present in samples analysed by colorimetric methods could result in over or under estimate of NH4-N in soils. All amino acids showed positive interference at zero NH4-N concentration. However, these amino acids in water and soil extracting solutions (2 M KCI and 0.5 M K2S04) showed negative interference at 1 mg l–1 NH4-N.