  De Concini and Procesi have defined the wonderful compactification of a
symmetric space X=G/H with G a semisimple adjoint group and H the subgroup of
fixed points of G by an involution s. It is a closed subvariety of a
grassmannian of the Lie algebra L of G. In this paper, we prove that, when the
rank of X is equal to the rank of G, the variety is defined by linear
equations. The set of equations expresses the fact that the invariant alternate
trilinear form w on L vanishes on the (-1)-eigenspace of s.
