We report on the spontaneous formation of an AlInN interlayer in plasma-assisted molecular beam epitaxial growth of InN on sapphire substrates. Under our growth conditions, and with an initial growth stage corresponding to the best attainable InN film quality, the AlInN layer has a thickness of ≈100 nm and an aluminum content of around 0.3. The layer possesses a good crystalline quality as manifested by transmission electron microscopy and X-ray diffraction (XRD) studies. No evidence of binary phase separation within the layer is found by XRD and optical absorption measurements. The energy gap bowing of the AlInN is well approximated by the expression for a disordered alloy. The possibility of AlInN to emit light is demonstrated by photo- and cathodoluminescence measurements.