Al1–xTixB2 particles extracted from Al–Ti–B alloys have been investigated by means of a transmission electron microscope equipped with an X-ray energy-dispersive spectroscopic analyser. The solid solubility Al1–xTixB2 was found to be unlimited (0 ≤ x ≤ 1). The composition of boride particles extracted from an Al–Ti–B alloy with a composition within the solid solubility range was widely spread, whereas it was close to TiB2 for Ti concentrations in excess of the TiB2 stoichiometry. The particle size varied from 0.02 to 2 μm. The largest particles were almost pure TiB2 or AlB2, but the small ones were solid solutions of varying compositions. Those small particles tended to form relatively large agglomerates. Due to impurities in the master alloys, vanadium was present in some boride particles, implying that we actually have an Al1–x(Ti1–yVy)xB2 solid solution (0 ≤ x + y ≤ 1). The y value was found to vary within the range 0 ≤ y ≤ 0.07. It was found that master alloys containing boride particles with a narrow size distribution and a composition close to TiB2 gives better grain refinement results than master alloys with wide distributions of particle size and composition.