The desorption kinetics from heterogeneous surfaces contain information on energy heterogeneity. Mathematical methods for extracting the energy distribution function from desorption kinetics are developed. The overall desorption kinetics are described as weighted averages of the local desorption kinetics from each energetically homogeneous zone forming the surface. Whatever the local kinetics, the energy distribution is determined from the overall kinetics by using the condensation approximation. The method is applied to two situations : the time-logarithm law observed in a lot of systems of practical interest in catalysis, and the time-power law observed in many relaxation kinetics of macromolecules.