We recently showed that poly(ADP-ribose) polymerase (PARP) inhibitors exert their cytotoxicity primarily by trapping PARP-DNA complexes in addition to their NAD(+)-competitive catalytic inhibitory mechanism. PARP trapping is drug-specific with olaparib exhibiting a greater ability than veliparib, while both compounds are equally potent catalytic PARP inhibitors. Here, we evaluated the combination of olaparib or veliparib with therapeutically relevant topoisomerase I inhibitors (camptothecin), alkylating agents (temozolomide), cross-linking agents (cisplatin) or topoisomerase II inhibitors (etoposide) at the cellular and molecular levels. We determined PARP-DNA trapping and catalytic PARP inhibition in genetically modified chicken lymphoma DT40 and human prostate DU145 and glioblastoma SF295 cancer cells. For camptothecin, both PARP inhibitors showed synergistic effect due to catalytic PARP inhibition rather than PARP trapping, indicating the value of combining either veliparib or olaparib since both are highly potent catalytic PARP inhibitors. On the other hand, for temozolomide, PARP trapping was much more relevant than catalytic inhibition of PARP, indicating that olaparib is more effective than veliparib in combination with temozolomide. As for cisplatin and etoposide, olaparib showed weaker combination effect than for camptothecin and temozolomide, which is consistent with the lack of involvement of PARP in the repair of cisplatin- and etoposide-induced lesion. Hence, we conclude that catalytic PARP inhibitors are highly effective in combination with camptothecins, whereas PARP inhibitors capable of PARP trapping are more effective with temozolomide. Our study provides new insights in combination treatment strategies of different PARP inhibitors.