Concentrated solutions containing 6-aminocaproic acid and the excipients citric acid and sorbitol have been studied at temperatures of 50 °C, 60 °C, 70 °C and 80 °C as well as at 20 °C. It has previously been reported that the commonly employed citric acid is a reactive excipient, and it is therefore important to thoroughly investigate a possible reaction between 6-aminocaproic acid and citric acid. The current study revealed the formation of 3-hydroxy-3,4-dicarboxy-butanamide-N-hexanoic acid between 6-aminocaproic acid and citric acid by high-resolution mass spectrometry (HRMS) and nuclear magnetic resonance spectroscopy (NMR). Less than 0.03% of 6-aminocaproic acid was converted to 3-hydroxy-3,4-dicarboxy-butanamide-N-hexanoic acid after 30 days of storage at 80 °C. Degradation products of 6-aminocaproic acid were also observed after storage at the applied temperatures, e.g., dimer, trimer and cyclized 6-aminocaproic acid, i.e., caprolactam. No reaction products between d-sorbitol and 6-aminocaproic acid could be observed. 3-Hydroxy-3,4-dicarboxy-butanamide-N-hexanoic acid, dimer and caprolactam were also observed after storage at 20 °C for 3 months. The findings imply that an oral solution of 6-aminocaproic acid is relatively stable at 20 °C at the pH values 4.00 and 5.00 as suggested in the USP for oral formulations. Compliance with the ICH guideline Q3B is expected.