This paper is the third in a series in which we study the superconvergence of finite element solutions by a computer-based approach. In [1] we studied classical superconvergence and in [2] we introduced the new concept of η%-superconvergence and showed that it can be employed to determine regions of least error for the derivatives of the finite element solution in the interior of any grid of triangular elements. Here we use the same ideas to study the superconvergence of the derivatives of the finite element solution in the interior of complex grids of quadrilaterals of the type used in practical computations. © 1994.