It is shown that double resonance experiments can establish the relative signs of all the coupling constants in AA′XX′ spin systems, and the technique is applied to the 19F–19F couplings in perfluorophenyl mercury compounds. Heteronuclear double resonance experiments are used to show that all the 199Hg–19F coupling constants in the compounds are positive, and that 3J(Hg–Ho) and 4J(Hg–Hm) are also positive in aryl mercury compounds. 199Hg chemical shifts are quoted.