Abstract: The relative signs of coupling constants in ABX system for isophytol were determined using two methods of double resonance:selective spin decoupling and spin tickling methods. Through this example a simple and clever method is illustrated by which the relationship between double resonance method, related diagram, a simple diagram about energy levels and relative signs of coupling constants in ABX system is linked up without precise calculation by means of intensities of all peaks. In this ABX system δ_A 5.0lppm, δ_B 5.04ppm, δ_x 5.85ppm,|J_AB|=1.5Hz,|J_AX=10.6Hz,|J_BX|=17.3Hz were determined. Signs of J_AB, J_AX,and J_BX are the same.

Key words: ³J_H-H coupling constants, Relative sign, ABX system, NMR, Double resonance, Iso-phytol