Retention behavior of diazines, consisted of pyrazines, pyridazines and pyrimidines, was studied at various column temperatures in reversed−phase liquid chromatography (RPLC). An increased retention of pyrazines was observed on several types of stationary phases including octadecylsilica (ODS) phases when column temperature was elevated, suggesting an abnormal temperature effect. Similar abnormal temperature effect on the retention was also observed for pyridazines and pyrimidines with an acetonitrile/water mobile phase at elevated column temperatures. On the other hand, no similar trend was found for these analytes with a methanol/water mobile phase. In addition, the retention for pyrazole, imidazole and their methyl derivatives was decreased with increasingthe column temperature regardless the type of organic modifier in the mobile phase. The results suggest that the abnormal temperature effect on the retention of diazines could be mainly induced by an unusual thermodynamic behavior between the stationary phase and the mobile phase containing acetonitrile.