Infrared spectra and frequency assignments of various isotopomers of formylperoxy radical HC(O)OO in solid argon are presented. The formylperoxy radical was produced through the discharge production of formyl radical followed by the association reaction of formyl radicals with molecular oxygen in solid argon. Quantum chemical calculations indicate that the observed spectrum can be assigned to formylperoxy radical with the O-O bond in a cis position relative to the C-H bond.