The purpose of this study was to obtain a better understanding of the effects of amino acids on the solid-state phase transitions of ammonium nitrate (AN). To this end, AN was combined with various amino acids in equimolar ratios and the phase transitions of the resulting mixtures were studied using differential scanning calorimetry (DSC) and in situ Raman spectroscopy. Compositional analysis was also conducted using X-ray powder diffraction (XRD), and the predicted stabilities of various molecular structures were assessed via quantum calculations. DSC and Raman results indicated that l-glycine and l-alanine inhibited the solid-state phase transitions of AN. Both XRD and calculation results suggested that AN reacts with these amino acids to form nitrate salts and that clusters are also generated by interactions between these compounds.