The phase diagrams for aminophenazone-urea (a), aminophenazone-thiourea (b) and aminophenazone-noxythioline (c), established by differential scanning calorimetry, transparency method and thermomicroscopy, revealed the presence of definite compounds (1-3). These compounds are formed during eutectic fusion for (a) and (c); the definite compound for (b) displays non-congruent fusion.