The electron paramagnetic resonance spectra of Mn(CsHsNO)6.X2 (X -CIO4, BF4 and NO3 ) doped in single crystals of isomorphous paramagnetic Co(C5 HsNO)6. X2 are studied at various temperatures. Zero-field splitting in all three crystals is axially symmetric and the magnitude olD is unusually large for an octahedral coordination polyhedron with all ligands identical. The sharp resonance of Mn(II) in the paramagnetic host observed at high temperatures is interpreted in terms of random modulation of the dipolar interaction between the guest Mn(II) and host Co(II) ions by the rapid spin-lattice relaxation of Co(II). The spin-lattice relaxation times of Co(II) ions at 300 K, estimated from the temperature dependent linewidth of the Mn(II) resonance, are 24 x 10 -12, 28 • l0 -12 and 23 x 10 12S in perchlorate, fluoborate and nitrate crystals respectively. The temperature dependence of the relaxation is of the form l/ (at + bt 5) and below 270 K the direct process dominates.