Infrared radiation has been observed in air as far as thirty body diameters ahead of ablating, hypervelocity bodies. This precursor is attributed to constituents of air such as CO2 and H2O, as well as C2 and CN contamination. The dominant mechanism for the precursor is thought to be resonance radiation due to photon flux absorption ahead of the shock. Precursor intensities were observed with infrared detectors and photomultipliers. Data from cylinders at velocities up to 5500 m/sec in air at from 20 to 75 mm Hg are presented. Some approximate formulas have been derived to account for precursor intensities. The data are compared with calculated estimates and fair agreement is observed. Water vapor is the most important contributor to the precursor while the effect of CO2 is small. The presence of C2 and CN as contaminants in the range apparatus is suggested.