The magnetic properties of Mn 1Àx Ni x Cl 2 Á 2H 2 O are examined by DC magnetization and susceptibility measurements on a wide range of compositions. The pure components are three-dimensional Heisenberg antiferromagnets, ordering at 6.7 K (MnCl 2 Á 2H 2 O) and at 7.2 5 K (NiCl 2 Á 2H 2 O). Each contains MC1 2 MC1 2 My chemical (and structural) chains, with intrachain exchange ferromagnetic and antiferromagnetic in the Ni and Mn materials, respectively. Interchain exchange is predominantly antiferromagnetic in each. The Curie and Weiss constants, in w M ¼ C=ðT2yÞ fits to high temperature data, vary regularly with composition. C(x) is essentially linear, and yðxÞ exhibits systematic curvature which is analyzed to show that unlike-ion exchange is antiferromagnetic and weaker than like-ion interactions. The form of the susceptibility evolves with composition: maxima in wðT) broaden and shift to lower temperature with admixture of either component, but beyond about 20% minority component such maxima are absent, with upturns appearing instead. T c descends rapidly with admixture from either composition extreme, but varies only weakly between x ¼ 0:30 and 0.70. Magnetization isotherms display a subtle composition dependence which suggests that the ordered state differs for intermediate compositions from those for the composition extremes. Although frustration in mixtures is likely, from competing (sign) intrachain interactions, it is not strong enough to yield magnetic irreversibilities or time dependences signaling a spin glass phase.