A series of new fluorescing 8‐(6‐hydroxyhexyl)isoalloxazine (=8‐(6‐hydroxyhexyl)benzo[g]pteridine‐2,4(1H,3H)‐dione) derivatives 4–13 were synthesized from 6‐[(6‐hydroxyhexyl)amino]uracil (2) with 1‐chloro‐4‐nitrosobenzene via 8‐chloro‐10‐(6‐hydroxyhexyl)isoalloxazine (3) and subsequent substitution of the Cl‐atom of 3 by various amines (Scheme). Analogously, 8‐substituted 10‐3‐[(2,2‐dimethyl‐1,3‐