Automated analyses for inorganic NH4 N are being performed on samples of widely differing organic and inorganic composition and this causes concern about the accuracy of the inorganic NH4 N determination. An automated phenol method for determination of inorganic NH4 N was compared with the steam distillation method for analysis of KCl extracts of forest floor samples. Amino acids added to the KCl extracts were found to contribute significant positive interference to the automated method. Modification of the automated method yielded results identical to steam distillation analyses. The original method was found to overestimate inorganic NH4 N in extracts of forest floor material by 17–26%.