Polycrystalline samples of the (AgInSe2)1–x(VSe)x system were prepared by the melt and anneal method. The anneal temperature was 900 K and the anneal time one month. The step composition was 0.1 and the weight of each sample approximately 1 g. Additionally, the composition x = 1/3 was also prepared, since these alloys have been reported as electronic, i.e. definite compounds exist in the diagram at precise values of composition, one of them x = 1/3. The stoichiometric relation of the samples was investigated by SEM technique. The experimental values, in average, are very close to the nominal values, all of them lying inside the interval of the experimental error (±5%). X-Ray Diffraction (XRD) and Differential Thermal Analysis (DTA) techniques were used for characterization of the alloy samples. Guinier photographs were obtained for all the samples and unit cell parameters were calculated using the available software for indexation. The diffraction patterns show sharp lines indicating good thermal equilibrium of the samples. Transition temperatures obtained from DTA measurements were manually obtained from the (T vs. T graph with the criteria that the transition occurs at the intersection of the base line with the slope of the thermal transition peak, as usually. From the analysis of the experimental results it was observed that the solid solubility of VSe in AgInSe2 is approximately 20%, i.e. the single-phase region exists in the composition range 0 ≤ x < 0.2, whereas for x > 0.2 at least two phases coexist. A schematic T-x phase diagram is proposed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)