The morphology of a bismaleimide (BMI) toughened with a thermoplastic hyperbranched aliphatic polyester (HBP) was studied by scanning electron microscopy (SEM). The effect of thermoplastic architecture, molecular weight, and end group on the size and arrangement of the dispersed phase was investigated and compared with the thermoset fracture toughness. SEM micrographs showed that higher molecular weight HBP formed roughly spherical dispersed domains of up to similar to 60 mu m, which contained BMI inclusions. Lower molecular weight HBP formed spherical dispersed thermoplastic domains, with diameters up to similar to 10 mu m with no BMI inclusions. A low molecular weight linear polyester with a repeat unit structure, which was similar to that of the HBP, was prepared and used as a control. Within error, BMI toughened with the linear control yielded the same fracture toughness as the best values obtained with HBP-modified BMI, but the morphology differed. The linear polyester phase separated into particles with a larger average diameter and also possessed some phase-inverted regions. End group effects were studied by modifying the hydroxy-terminated HBP to unreactive nitrophenyl, phenyl, and acetyl end groups. The nitrophenyl-terminated HBP did not phase separate from the thermoset, whereas the nonpolar phenyl-and acetyl-terminated HBP phase separated to form small (less than or equal to 1 mu m and similar to 2 mu m, respectively) spherical domains. Some comparisons were made to other results with HBP thermoplastics in BMI and epoxy thermosets.