In the two title optically active tetrahydroisoquinoline derivatives, namely 3-hydroxymethyl-4-phenyl-1,2,3,4-tetrahydroisoquinolin-2-ium bromide methanol hemisolvate, C(16)H(18)NO(+).Br(-).0.5CH(3)OH, (IIb), and 2-formyl-3-hydroxymethyl-4-phenyl-1,2,3,4-tetrahydroisoquinoline, C(17)H(17)NO(2), (III), the absolute configurations have been confirmed as 3R,4R by structure refinement using Bijvoet-pair reflections. The hydroxymethyl and phenyl groups in (IIb) are oriented in equatorial and pseudo-equatorial positions, respectively, whereas in (III), the corresponding groups are in axial and pseudo-axial positions, respectively; the hydroxymethyl and phenyl groups are trans with respect to one another in both structures. The heterocyclic rings in (IIb) and (III) adopt envelope conformations inverted with respect to each other. In both structures, the molecules are linked through hydrogen bonds.