The reaction of CH₃CHOO with HC(O)OH has a large rate coefficient so that it might play a significant role in the formation of secondary organic aerosols (SOA) in the atmosphere. We investigated the detailed mechanism and kinetics of the reactions of Criegee intermediate anti- and syn-CH₃CHOO with HC(O)OH with a step-scan Fourier-transform infrared spectrometer by recording time-resolved absorption spectra of transient species and end products produced upon irradiation at 308 nm of a flowing mixture of CH₃CHI₂/O₂/HC(O)OH at 298 K and 60 Torr. Thirteen bands of hydroperoxyethyl formate [HC(O)OCH(CH₃)OOH, HPEF], the hydrogen-transferred adduct of CH₃CHOO and HC(O)OH, were observed. Careful analysis deconvoluted these bands into absorption of three conformers of HPEF: a transient HPEF (P2*/P3*), a more stable open-form HPEF (mainly P2), and a stable intramolecularly hydrogen-bonded HPEF (mainly P1). At a later period, the end-product formic acetic anhydride [CH₃C(O)OC(O)H, FAA], a dehydrated product of HPEF, was observed; this end-product is the same as that observed in CH₂OO + CH₃C(O)OH. Theoretical calculations on the reaction pathway scheme were performed to elucidate these reaction paths. Syn-CH₃CHOO + HC(O)OH produced conformers P2*/P3* initially, followed by conversion to conformers P2, whereas anti-CH₃CHOO + HC(O)OH produced conformers P2 and P1 directly. We derived a rate coefficient for the reaction CH₃CHOO + HC(O)OH to be k = (2.1 ± 0.7) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 298 K and 40–80 Torr; the rate coefficient appeared to show insignificant conformation-specificity. We also found that FAA was produced mainly from the dehydration of the open-form HPEF (P2) with a rate coefficient k = (1420 ± 70) s⁻¹; the intramolecularly hydrogen-bonded HPEF (P1) is stable.