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Group VB and VIB M–Si systems are considered to show an interesting pattern in the diffusion of components with the change in atomic number in a particular group (M = V, Nb, Ta or M = Mo, W, respectively). Mainly two phases, MSi₂ and M₅Si₃ are considered for this discussion. Except for Ta–silicides, the activation energy for the integrated diffusion of MSi₂ is always lower than M₅Si₃. In both phases, the relative mobilities measured by the ratio of the tracer diffusion coefficients decrease with an increasing atomic number in the given group. If determined at the same homologous temperature, the interdiffusion coefficients increase with the atomic number of the refractory metal in the MSi₂ phases and decrease in the M₅Si₃ ones. This behaviour features the basic changes in the defect concentrations on different sublattices with a change in the atomic number of the refractory components.