Wetting of chemically heterogeneous surfaces is modeled using a phase field theory. We focus on a chemically heterogeneous surface composed of squares of one component material embedded in another. Unlike previous studies where the component materials were characterized only by an equilibrium contact angle, in this paper each of the component materials is constitutively allowed to exhibit hysteresis. Using this approach, we investigate the effect of heterogeneity length scale on observed macroscopic behavior. Cassie theory is found to be applicable only in the limit of vanishing length scale. For surfaces with a finite heterogeneity length scale, the advancing and receding contact angles deviate from Cassie theory. We find that this deviation and its length scale dependence are asymmetric and depend on the wetting properties of the embedded material relative to the contiguous substrate. © 2009 American Chemical Society.