Epithelial to mesenchymal transition (EMT) is a process by which an epithelial cell alters its phenotype to that of a mesenchymal cell and plays a critical role in embryonic development, tumour invasion and metastasis and tissue fibrosis. Transforming growth factor-β1 (TGF-β1) continues to be regarded as the key growth factor involved in driving EMT however recently tumour necrosis factor α (TNFα) has been demonstrated to accentuate TGF-β1 driven EMT. In this study we investigate how various signalling pathways contribute to this accentuated effect. A549 cells were treated with TGF-β1 (10 ng/ml), TNFα (20 ng/ml) or a combination of both for 72 h and EMT assessed. The effect of selective inhibition of the SMAD, MAPK and NF-κB pathways on EMT was assessed. A549 cells treated with TGF-β1 downregulate the expression of epithelial markers, increase the expression of mesenchymal markers, secrete matrix-metalloproteinases and become invasive. Significantly, TGF-β1 driven EMT is accentuated by co-treatment with TNFα. SMAD 3 inhibition attenuated TGF-β1 driven EMT but has no effect on the accentuation effect of TNFα. However, inhibiting IKKβ blocked both TGF-β1 driven EMT and the accentuating action of TNFα. Inhibiting p38 and ERK signalling had no effect on EMT. TNFα accentuates TGF-β1 driven EMT in A549 cells via a SMAD 2/3 independent mechanism involving the NF-κB pathway independent of p38 and ERK 1/2 activation.