The tautomerism of 5-fluorobenzofuroxan was studied by the proton magnetic resonance at 60 and 100 MHz. The rate of intramolecular rearrangement between 5-fluorobenzofuroxan (ABCX) and 6-fluorobenzofuroxan (A'B'C'X') was obtained by analyzing the line shapes of proton magnetic resonance signals. The theoretical spectra were calculated by means of a computer program based on the density matrix theory of the nonmutual exchange developed by Johnson, and the theoretical spectra showed a good agreement with the experimental line shapes in the temperature range from -78°to +30°. Activation parameters for the intramolecular rearrangement were found to be ⊿H^≐̸=5∼10kcal/mol and ⊿S^≐̸=-33∼-17 cal/deg·mol.