We shall define the relative class of a pair of Lie algebroids. For a Lie subalgebra of a Lie algebra, the relative class is the obstruction to the existence of an invariant measure on the corresponding homogeneous space. We shall define the modular class of a Lie algebroid with a twisted Poisson structure, and prove that it can be expressed in terms of generators of the associated Gerstenhaber algebra. Then, we shall show how to interpret this class as the relative class of a pair of Lie algebroids.