  A reaction-diffusion equation on a family of three dimensional thin domains,
collapsing onto a two dimensional subspace, is considered. In \cite{\rfa pr..}
it was proved that, as the thickness of the domains tends to zero, the
solutions of the equations converge in a strong sense to the solutions of an
abstract semilinear parabolic equation living in a closed subspace of $H^1$.
Also, existence and upper semicontinuity of the attractors was proved. In this
work, for a specific class of domains, the limit problem is completely
characterized as a system of two-dimensional reaction-diffusion equations,
coupled by mean of compatibility and balance boundary conditions.
