The characteristics of wetting in polymer blends are investigated by a self-consistent-field theory. A symmetrical system is chosen: the interface between two homopolymeric liquids A and B is wetted by a third homopolymer C which is equally insoluble in both liquids. All components have the same molecular weight (N-A=N-B=N-C=N=10 or 100). The emphasis of this study is on the wetting transitions induced by varying the interactions between the components. Cahn's argument, which predicts complete wetting near the critical temperature of two system components, is verified in this context. We show that it is necessary to consider the effective interaction parameters chi (eff)(AC)=chi (eff)(BC) to verify Cahn's argument. Since we vary the solubility of C (given by chi (AC)=chi (BC)) and the thickness of the AB interface (determined by chi (AB)) independently, we have a two-dimensional parameter space. In this parameter space we can distinguish three regimes representing wetting transitions with different characteristics. One of these regimes indeed shows Cahn-type transitions. A key observation is that the wetting transitions near the simultaneous critical point of mixtures A/C and B/C are of a second-order type. A second regime in the parameter space represents wetting transitions which are understood from the high surface tension of the AB interface. In many cases these wetting transitions are also of a second-order type, but become first order when N chi (AB)>8. In the third regime we find what might be called "pseudowetting:" from inspection of the adsorption isotherms it follows that C seems to wet the AB interface, but with increasing amount of C in the system, the wetting layer is suddenly destroyed. The reason for this is clear. Here, the apparent wetting point chi (wet)(AC) is close to chi (eff,crit)(AC), but chi (AB)< chi (AC)=chi (BC), the wetting film is unstable. As a result, while at first Cahn's argument seems to be fulfilled, it eventually fails in this region of the parameter space. (C) 2001 American Institute of Physics.