Herein, we report the graded electronic band gap along the axis of individual heterostructured WZ-ZB InAs/InSb_0.12As_0.88 nanowires. Resonance Raman imaging has been exploited to map the axial variation in the second excitation gap energy (E₁) at the high symmetry point (L point) of the Brillouin zone. We relate the origin of the observed evolution of the gap energy to the fine tuning of the alloy composition from the tip towards the interface of the nanowire. The electronic band structures of InAs, InSb and InSb_xAs_1−x alloy systems at x = 0.125, 0.25, 0.50, 0.75 and 0.875, using the all electron density functional theory code Wien2k, are reported. The measured band gap along the axis of the InAs/InSb_0.12As_0.88 nanowire is correlated with the calculated gap energy at the A point and the L point of the Brillouin zone for InAs and InSb_0.125As_0.875, respectively. We draw a one-to-one correspondence between the variation of the E₁ gap and the fundamental E₀ gap in the calculated electronic band structure and propose the graded fundamental gap energy across the axis of the nanowire.