By making use of a novel diastereotopicity probe, namely C(CF3)2OH, it has been possible to measure by very low temperature 19F NMR spectroscopy the elusive aryl–aryl rotation barriers of biphenyls bearing an OH or F group in one ortho position. The experimental values (5.4 and 4.4 kcal mol−1, respectively) are matched by those from ab initio calculations (5.3 and 4.3 kcal mol−1, respectively).