  We consider the (scalar) gradient fields $\eta=(\eta_b)$--with $b$ denoting
the nearest-neighbor edges in $\Z^2$--that are distributed according to the
Gibbs measure proportional to $\texte^{-\beta H(\eta)}\nu(\textd\eta)$. Here
$H=\sum_bV(\eta_b)$ is the Hamiltonian, $V$ is a symmetric potential, $\beta>0$
is the inverse temperature, and $\nu$ is the Lebesgue measure on the linear
space defined by imposing the loop condition
$\eta_{b_1}+\eta_{b_2}=\eta_{b_3}+\eta_{b_4}$ for each plaquette
$(b_1,b_2,b_3,b_4)$ in $\Z^2$. For convex $V$, Funaki and Spohn have shown that
ergodic infinite-volume Gibbs measures are characterized by their tilt. We
describe a mechanism by which the gradient Gibbs measures with non-convex $V$
undergo a structural, order-disorder phase transition at some intermediate
value of inverse temperature $\beta$. At the transition point, there are at
least two distinct gradient measures with zero tilt, i.e., $E \eta_b=0$.
