A series of frequency sweep nuclear magnetic double resonance experiments have been performed on 2-hydroxy-3-methoxybenzaldehyde; the strong irradiation field was applied to various parts in the ring proton band and the effects were observed in the hydroxyl band. The transition frequencies and transition probabilities in the double resonance spectra were calculated by numerical diagonalization of the double resonance Hamiltonian of the ABCX subsystem composed of the three ring protons and the hydroxyl proton. The large number of closely spaced transitions (eight transitions within an interval of less than 0.8 cps) necessitated a more detailed evaluation of the line-shapes. Superposition of eight Lorentzian resonance lines produced theoretical spectra in very good agreement with the observed ones and it was found that changes in transition frequencies amounting to a few hundredths of a cps produced significant changes in line-shape. The fact that such marginal effects can be employed for the evaluation of the spectra is attributed mainly to the high reproducibility of the experimental line-shape as obtained with NMR spectrometers employing internal proton stabilization. On the basis of the experiments one may deduce 1. (i) that the resolved spin coupling J OH-4 = 0.62 cps carries the same sign as the ring proton couplings and 2. (ii) that there exists an additional unresolved spin coupling J OH-6 = 0.13 ± 0.05 cps which also carries the same sign as the ring proton spin couplings. The general problems of sign determination of spin couplings and of the estimation of unresolved spin couplings are briefly discussed.