The unit cell parameters of 19 samples of K-feldspar of the Argentera granites were determined from X —ray powder data, through a least-squares refinement computer program. The values obtained were plotted on diagrams available from the literature to determine the structural state and the composition, and to detect “anomalous” unit cells of the above mentioned feldspars. The principal result are as follows: a) The potassium-rich phase of these microperthites is always higly ordered in terms of Al : Si distribution. Most samples are as highly ordered as natural “maximum” microclines of the lowest structural state, b) The slight “anomalies” observed in the dimensions of the unit cell of some samples cannot be attributed to strain effects in cryptoperthites. It is suggested that they may be a result of the presence of the H₃OAlSi₃O₈ end member in solid solution, c) All the samples examined have an Ab content in solid solution ranging from 4% to 0%. d) The variations of structural state and of Ab content in solid solution noted in the 19 samples examined are extremely slight and are not related to the type of granite which contains the microperthites. The results have provided indications about the chemical and physical conditions which governed the crystallisation of microperthites in the Argentera Massif granites.