The fluorescence excitation spectrum of the A 1A" state of formic acid vapor has been measured in the 37 200-40 300-cm-1 region under various magnetic field strengths up to 12 kG. It is found that the intensity of the spectrum is considerably reduced by external magnetic fields, the maximum reduction being 45%. The pressure and field strength dependences of the magnetic fluorescence quenching suggest that the quenching arises from intramolecular acceleration of intersystem crossing due to magnetically induced spin decoupling in the triplet state. From the excess energy dependence of the magnetic quenching, it has been found that the degree of quenching increases with increasing vibrational excess energy from the vibrationless level (37 432 cm-1) of the A state. However, a sudden decrease in the quenching efficiency is observed above the 3(1) level (approximately 38 60O cm-1), which can be attributed to predissociation.