1-(5-(Benzyloxy)-2-(oxiran-2-ylmethoxy)phenyl)-3-phenyl-d5-propan-1-one is the labeled analogue of 1-(5-(Benzyloxy)-2-(oxiran-2-ylmethoxy)phenyl)-3-phenyl-propan-1-one (CAS 118635-89-5), a reactant used in the synthesis of 5-hydroxy and 5-benzyloxy analogs of the antiarrhythmic and multidrug resistance (MDR) modulating drug propafenone (P757500).