Absorption measurement of a gas-phase iron oxide (FeO) electronic spectrum is reported for the first time. FeO was obtained in a methane/oxygen/nitrogen flame doped with iron pentacarbonyl and monitored via the 611-nm band of the orange system with intracavity laser absorption spectroscopy. The measured spectral range extends from 16,250 to 16,450 cm−1 for the appropriate intracavity tuning element position. The presented measurements demonstrate sensitive detection of the gas-phase precursor, playing a key role in flame synthesis of iron oxide nanoparticles. The sensitivity of the intracavity technique does not depend on broadband cavity losses and allows conducting measurements in a highly scattering, particle-laden flame environment.