We report the fully analyzed high-resolution spectrum of the carboxylic acid dimer (HCOOH)2 in the gas phase. High-resolution IR spectra in the region of the antisymmetric C-O stretch vibration have been recorded at 1221.0-1226.7 cm(-1). The data could be fit within experimental uncertainty to a rigid rotor Watson S-reduced Hamiltonian. The vibrational frequency of the C-O vibration is 1225.3430(3) cm(-1). On the basis of the measurement of a tunneling splitting of 0.0158(4)cm(-1) for the lower state and 0.0100(3) cm(-1) for the upper state, we determine a proton-transfer time of 1.0 and 1.7 ns for the ground and the vibrationally excited state, respectively.