The syntheses and characterization of several complexes of Mn(II) with cyclen derivatives having variable numbers of pendant N-acetic acid or N-acetamide arms are reported. X-ray crystallographic results are presented for Mn(DOTAM)Cl-2 center dot 2H(2)O (monoclinic C2/c, a = 18.5798(15), b = 13.6006(11), c = 10.5800(8) angstrom, beta = 110.490(1), Z = 4), [Mn(DO3AM)][MnCl4] center dot EtOH (monoclinic P2(1)/n, a = 8.366(8), b = 19.483(2), c = 16.3627(16) angstrom, 99.254(2)degrees, Z = 4), and Mn(H(2)DOTA) (monoclinic C2/c, a = 16.374(3), b = 6.6559(13), c = 16.750(3) angstrom, beta = 98.381(3)degrees, Z = 4), which exhibit 8-, 7-, and 6-coordinate Mn(II), respectively. H-1 relaxivity data in water at 20 MHz and 37 degrees C is presented and interpreted in terms of a mechanism involving transient binding of water in an associative intermediate. Relaxivity studies in mixed water/methanol solvents are consistent with this interpretation.