Carbon-to-nitrogen ratio (C:N) has frequently been shown to be a good predictor of the speed of organic residue decomposition and N mineralization in soil. While this relationship appears to work well for complex organic materials (e.g. plant litter), its applicability to smaller organic substrates containing N remains unknown. Here we evaluated whether the intrinsic properties of amino acids and peptides could be used to predict their rate of microbial uptake and subsequent N mineralization. In an agricultural grassland soil we found that C:N, molecular weight, aromaticity and sulphur content provided poor indicators of amino acid bioavailabilityand subsequent NH4+ release into soil. We therefore hypothesize that the position of amino acids along microbial biosynthetic pathways together with internal demand for individual amino acids rather than their C or N content is the primary determinant of N mineralization.