The E/Z isomerization process of a uracil-azobenzene derivative in which the nucleobase is conjugated to a phenyldiazene tail is studied in view of its ability to form triply H-bonded complexes with a suitably complementary 2,6-diacetyla-mino-4-pyridine ligand. UV-vis and H-1 NMR investigations of the photochemical and thermal isomerization kinetics show that the thermal Z -> E interconversion is 4-fold accelerated upon formation of the H-bonded complex. DFT calculations show that the formation of triple H-bonds triggers a significant elongation of the N = N double bond, caused by an increase of its pi(g)* antibonding character. This results in a reduction of the N = N torsional barrier and thus in accelerated thermal Z -> E isomerization. Combined with light-controlled E -> Z isomerization, this enables controllable fractional tuning of the two configurational isomers.