The new reduction reaction of the hydroxy groups of 4-hydroxy-4-phenyl-1,2,3,4-tetrahydroisoquinolines (1) to the corresponding alkanes (2) with mineral and Lewis acids is reported. A stereoselective intermolecular hydride shift mechanism of the reduction was proved by reaction of the deuterated derivateives (14 and 15) of 1a with 10N HCl-C2H5OH and BBr3 in CH3CN.