Matrix samples of CH2N2/O2/Ar were photolyzed at 8 °K. Subsequent slow warming produced the previously observed O2 A′ 3Δu→X 3Σg- thermoluminescence and two new progressions, one at 3100-3600 Å and one at 3900-4900 Å. With 18O2, the 3100-3600 Å progression displayed isotopic shifts that allow two possible υ″ vibrational assignments to give ν00=34 730 or 33 340 (±40) cm-1 and ωe″=1390±15 cm-1. Similarly, the 18O2 shifts in the 3900-4900 Å progression lead to two possible υ″ assignments to give ν00=26 230 or 25 100 (±60) cm-1 and ωe″=1124±40 cm-1. These spectroscopic parameters point to assignments of the a 3A″→X 1A′ and A′ 1A′→A 1A″ transitions of HCOOH for the 3100-3600 and 3900-4900 Å progressions, respectively. These are the first observations of such transitions of formic acid and are attributed to the CH2(3B1)+O2(3Σ g-) reaction in the cryogenic matrix.