De Concini and Procesi have defined the wonderful compactification $\overline{X}$ of a symmetric space $X=G/G^{\sigma }$ where $G$ is a complex semisimple adjoint group and $G^{\sigma }$ the subgroup of fixed points of $G$ by an involution $\sigma$. It is a closed subvariety of a Grassmannian of the Lie algebra $𝔤$ 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 $𝔤$ vanishes on the $(-1)$-eigenspace...