Enantioselectivity can nonetheless be deduced from oxidations of enantiopure isotopomers (Sa)-D2-3a and (Ra)-D2-3a, which were made with 95% deuterium incorporation by methods described in the Supplementary Information and whose absolute configuration was determined as described in the SI. As depicted in Figure , oxidation of (Sa)-D2-3a generates monoaldehyde 4a in which the deuterium label (CD2 or CD) is distributed between the aldehydic CDO and benzyl alcoholic CD2OH positions in a ratio of 93:7 ( 1 H NMR spectroscopy). Conducting the experiment with the isotopomer, (Ra)-D2-3a, gives a ratio of deuterium incorporation at the aldehydic and benzylic positions of 5:95. The enantioselectivity of the oxidation, rox(Sa)/rox(Ra), determines the ratio of deuterium incorporation (e.g., for (Sa)-D2-3a, rox(Sa)/rox(Ra) = aldehydic signals labelled with D / benzylic signals labelled with D2), where the difference between the ratios and the conversions observed for the two isotopomers arises from the kinetic isotope effect (KIE) of the oxidation. Together, these experiments indicate that oxidation of unlabelled motor 3a to monoaldehyde 4a proceeds with an ee that falls in the range 85.7 ± 6.1% ee to 89.5 ± 2.7% ee, confirming that rox(Sa)/rox(Ra) ≠ 1. This experiment provides the first direct evidence of directional motion in an operational single bond rotary motor. The experiments detailed in Figure , the confirmation of a hierarchy of rates, and this direct observation of enantioselectivity in one of the constituent steps of the cyclic redox reaction network, confirm that under the standard reaction conditions, biphenyl 3a undergoes continuous net directional rotation about the C-C single bond. With the oxidation of 3a to 4a proceeding at a rate of rox(Ra,Sa) = 7.63 x 10 -4 mM.s -1 and assuming an average enantioselectivity of 87.6% ee, we determine that for a statistically relevant population of motor 3a, the mean number of 360° rotations after 48 h of operation will be 2.48 (Figure ). The rotational frequency of biphenyl motor 3a could be increased by optimising the rate and/or enantioselectivity of the oxidation, and by developing a cyclic redox reaction network comprising stereoselective oxidation and reduction pathways. Cyclic reaction networks comprising enantioselective biocatalytic reactions for both oxidation and reduction have been reported for the deracemization of point chiral alcohols by Kroutil and co-workers. In summary, rotary motor 3a undergoes autonomous directional rotary motion about a C-C single bond, powered by a biocatalytic cyclic redox reaction network. Our studies confirm 3a as a substrate for the cyclic redox reaction network; a kinetic analysis establishes that the system conforms to a hierarchy of rates required for rotary rather than oscillatory motion; deuterium isotopomer studies confirm the stereoselectivity of the biocatalytic oxidation of 3a and allow, for the first time, the directionality of rotary motion of a functioning single bond molecular motor to be confirmed. Through this report of the first redox driven single bond rotary motor, biocatalysis emerges as a powerful tool for the design and development of autonomously operating chemically fuelled molecular motors.