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The temperature dependence of magnetic properties of a series of aluminium-substituted cobalt ferrite, with the general composition of CoAlxFe2-xO4 (x = 0, 0.1, 0.2, 0.5), has been studied. It was observed that the magnetization, at an applied field of mu0 H = 5 T and a temperature of 10 K, does not change significantly for small amounts of Al, then reduces sharply for larger amounts of Al. Magnetic hysteresis loops were measured over a field range of -5 T les mu0H les 5 T at temperatures between 10 and 400 K. The high field regions of these loops were modelled using the Law of Approach to saturation, which can be used to calculate the magnetocrystalline anisotropy using its description of the processes of rotation of domain magnetizations against anisotropy and forced magnetization. It was found that the first-order cubic anisotropy of these materials decreased with increasing temperature. It was also observed that the anisotropy of these materials decreased with increasing Al-content at a given temperature.