The phase relations in the Al-Ti-B system are of great importance for the manufacturing of Al/TiB2 composites. To describe the phase relations, the relations in the binaries Al-B, Ti-B and Al-Ti must be known. The description of the binary Al-B system has been found to be poor. The Al-rich side of the Al-B phase diagram was studied using DSC, on alloys ranging from 0.5 up to 6.4 at ·% B. The Al-rich part of the phase diagram was assessed using the experimental data and data found in the literature. The eutectic temperature was found to agree well with the literature. However, the peritectic temperature was found to be 914°C. Above the peritectic temperature a metastable primary precipitation of AlB2 below the AlB12 liquidus line was suggested. This type of degenerate peritectic reaction is similar to what has been found in Fe-based materials. This behaviour also explains the difficulty in the determination of the peritectic point by experimental means. © 2001 Elsevier Science Ltd. All rights reserved.