The lattice constant of the A-(C) interstitial solid solutions is calculated, and its dependence on the solution composition is determined for arbitrary concentration cC of the C atoms and their arbitrary distribution over interstices of different types. For c c ≪1, a linear dependence of the lattice constant on cC is established, which is also observed experimentally. The influence of temperature redistribution of interstitial atoms over interstices of different types, leading to a specific nonlinear temperature dependence of the lattice constant, is investigated.