Reaction of manganese(II) perchlorate hexahydrate with a methanol solution of 1-thia-4,7-diazacyclononane ([9]-aneN(2)S) resulted in the isolation of the manganese(II) complex [Mn([9]aneN(2)S)(2)](ClO4)(2). The X-ray structure of this complex is reported : crystal system orthorhombic, space group Pbam, No. 55, a = 7.937(2) Angstrom, b = 8.811(2) Angstrom, c = 15.531(3) Angstrom, Z = 2, R = 0.0579. The complex is high spin (S = 5/2) with an effective magnetic moment (mu(eff)) 5.82 mu(B) at 298 K and 5.65 mu(B) at 4.2 K. Computer simulation of the Q-band EPR spectrum of [Mn([9]aneN(2)S)(2)](ClO4)(2) yields g = 1.99 +/- 0.01, D = 0.19 +/- 0.005 cm(-1), and E/D = 0.04 +/- 0.02. For the analogous hexaamine complex [Mn([9]aneN(3))(2)](ClO4)(2) ([9]aneN(3) = 1,4,7-triazacyclononane) analysis of the EPR spectra produced the following values : g = 1.98 +/- 0.01, D = 0.09 +/- 0.003 cm(-1), and E/D = 0.1 +/- 0.01. The spin Hamiltonian parameters for [Mn([9]aneN(2)S)(2)](ClO4)(2) derived from the EPR spectra produced a good fit to the magnetic susceptibility data.