The 351.1 nm photoelectron spectrum of the peroxyformate anion has been measured. The photoelectron spectrum displays vibronic features in both the (X) over tilde (2)A '' ground and (A) over tilde (2)A' first excited states of the corresponding radical. Franck-Condon simulations of the spectrum show that the ion is formed exclusively in the trans-conformation. The electron affinity (EA) of the peroxyformyl radical was determined to be 2.493 +/- 0.006 eV, while the term energy splitting was found to be 0.783(-0.020)(+0.060) eV. Extended progressions in the C-OO (973 +/- 20 cm(-1)) and O-O (1098 +/- 20 cm(-1)) stretching modes are observed in the ground state of the radical. The fundamental frequency of the in-plane OCO bend was found to be 574 +/- 35 cm(-1). The gas-phase acidity of peroxyformic acid has been determined using an ion-molecule bracketing technique. On the basis of the size of the trans- to cis- isomerization barrier, the measured acidity was assigned to the higher energy trans-conformer of the acid. The gas-phase acidity of the lower energy cis-conformer of peroxyformic acid was found from the measured acidity for the trans-form and a calculated energy correction: Delta(a)G(298)(cis-peroxyformic acid) = 346.8 +/- 3.3 kcal mol(-1) and Delta H-a(298)(cis-peroxyformic acid) = 354.4 +/- 3.3 kcal mol(-1). Using a negative ion EA/acidity thermochemical cycle, the O-H bond dissociation energy (D-0) values of the trans- and cis-conformers of peroxyformic acid to form the trans-radical were determined to be 94.0 +/- 3.3 and 97.1 +/- 3.3 kcal mol(-1), respectively. The heat of formation (Delta H-f(298)) of the trans-peroxyformyl radical was found to be -22.8 +/- 3.5 kcal mol(-1).