We propose a method to calculate the temperature dependence of Heisenberg exchange coupling constants J_ij. Within the formalism of disordered local moments (DLM), the magnetization and the J_ij are computed from first principles for any concentration of c of the magnetic constituents. The exchange coupling constants are then used in Monte Carlo (MC) simulations to compute the temperature dependence of the magnetization for the given c. By comparing the magnetization from DLM calculations and from MC simulations we obtain a mapping of temperature versus concentration and eventually temperature-dependent J_ij. The approach which is applied to bulk Fe and Co can for example improve critical exponents.