Magnesium anodising for corrosion and wear protection purposes currently attracts significant interest; however the mechanism of oxide film formation on this metal is not well understood. In this work, anodic behaviour of magnesium in alkaline solutions was investigated using chrono-amperometric methods. A model of instantaneous three-dimentional nucleation and growth was applied to describe complete voltastatic dependencies obtained when anodising Mg in electrolytes with different concentrations of KOH, Na2SiO3 and NaAlO2. The main parameters of kinetic equation, such as nucleation rate constant, and diffusion coefficient, were calculated and related with the nature of anodic oxides that is affected by different electrolyte additives. A good agreement achieved between the experimental and the calculated data confirms the validity of the proposed model which can be particularly useful in studies of anodic processes associated with precipitation of insoluble products, such as oxide/hydroxide compounds and salts comprising both metal substrate and electrolyte-derived species.