The synthesis of gold nanorods is commonly performed via seed-mediated growth processes, where typically tetrachloroaurate(III) is reduced with ascorbic acid in the presence of a surfactant (CTAB). However, in most cases nanorods with average aspect ratios below 4 are obtained, and reproducible processes which lead to average aspect ratios above 10 are widely missing. Using a modified seed-mediated method, we show that the average aspect ratio of gold nanorods increased from 8 to 19 when H2O was replaced by D2O as a solvent. The aspect ratio can be varied between these values by appropriate H2O–D2O mixtures. The increase in aspect ratio in the presence of D2O is established rather by an increase in nanorod length than in diameter. Apparently, the increase in length is not caused by an influence of D2O on the seed nanoparticles but is associated with the growth solution. It appears that in D2O a slower reduction of gold(III) species by ascorbic acid, with involvement of an O–D bond (or an O–H bond in H2O) in the rate determining step, is the origin of higher selectivity in the growth processes which lead to the different morphologies in nanoparticle formation.