The mixing behavior of the hole- and electron-transporting materials in bulk heterojunction (BHJ) organic photovoltaic (OPV) blends plays a key role in determining the nanoscale morphology, which is believed to be a decisive factor in determining device performance. We present a systematic investigation of the mixing behavior between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) in model multilayer structures. The bilayer structures are composed of amorphous PCBM that is mechanically laminated to different P3HT layers with varying degrees of crystallinity. We find that mixing is significantly decreased as the crystallinity of P3HT is increased. The mixing behavior can be explained as resulting from (1) nearly complete miscibility of PCBM with amorphous P3HT (based on our results from regiorandom P3HT) and (2) the existence of tie chains between crystalline P3HT domains that restrain the swelling of the P3HT layer by PCBM. We also introduce a unique PCBM-P3HT-PCBM trilayer structure where one of the PCBM layers is highly crystalline. The crystalline PCBM dramatically alters the mixing behavior. Initial mixing of the amorphous PCBM into P3HT is followed by rapid cold crystallization at the crystalline PCBM layer, which depletes the PCBM in the P3HT layer. These bilayer and trilayer experiments illustrate that mixing of P3HT and PCBM is influenced by multiple factors, such as the semicrystalline nature of P3HT (overall crystallinity, characteristics of amorphous chains) and phase (amorphous or crystalline) of the PCBM.