Tc-99m and Tc-99 complexes of PnAO (propylene diamine dioxime) ligands monosubstituted in the B-position [PnAO-6-R] were prepared and studied. Ligands substituted with an alkyl group or with no substituent (R = H, CH3, or CH2CH(CH3)(2)), gave only one Tc complex. However, for several other nonalkyl substituents (R = COOCH3, OH, OCH3, OCH2CH3, F, CN, NHCOCH3, and NHCOCH2CH3), two Tc complexes A and B were formed. Products A and B were assigned to the anti and syn TcO(PnAO-6-R) species, respectively, based on H-1 NMR results. X-ray structure analyses supported these assignments. The A (anti) isomer of. TcO(PnAO-6-OH) had the chemical formula TcC13H25N4O4 and crystallized in an orthorhombic system with space group P2(1)2(1)2(1) and Z = 4; a = 12.744(2) Angstrom, b = 13.591(2) Angstrom, c = 9.976(2) Angstrom. The B (syn) isomer of TcO(PnAO-6-CN) had the chemical formula TcC14H24N5O3 and was a 1:4 mixture of two monoclinic polymorphs: individual rectangular prisms (space group P2(1)/c, Z = 4) and clusters of intergrown twinned rectangular rods (space group Cc, Z = 8). For the prisms, a 12.457(1) Angstrom, b = 13.932(1) Angstrom, c 10.336(1) Angstrom, and for the rods, a = 31.344(5) Angstrom, b = 6.993(1) Angstrom, c = 21.657(2) Angstrom. The syn and anti isomers interconverted in the presence of water; nonequilibrium mixtures of epimers remained unchanged under dry conditions. The HPLC behavior under reversed phase conditions was consistent with on-column interconversion (poor resolution), whereas the two isomers were cleanly resolved under drier normal phase conditions. An oro inversion mechanism involving trans water attack is proposed for the interconversion process. Water also influenced the position of equilibrium of the two isomers. The syn isomer was stabilized ill water relative to the anti isomer.