D.C. susceptibility and zero field Mössbauer spectroscopy measurements are used to characterize the low-temperature magnetism of (\alpha-\mathrm{Rb}{2} \mathrm{FeF}{5} \cdot \mathrm{H}{2} \mathrm{O}) and (\alpha-\mathrm{Cs}{2} \mathrm{FeF}{5} \cdot \mathrm{H}{2} \mathrm{O}). The compounds are rigorously isotypic quasi-1D Heisenberg magnets based on hydrogen bonded chains of (-\left{\mathrm{FeF}{5} \mathrm{H}{2} \mathrm{O}\right}^{2-}) - pseudo-octahedra with intervening (\mathrm{Rb}^{+})and (\mathrm{Cs}^{+})cations that enhance the low dimensionality. The 1D antiferromagnetic effects appear comparable, (T_{\gamma \max }(1 \mathrm{D})) (\sim 5 \mathrm{~K}), while their (3 \mathrm{D}) ordering temperatures are quite different, (T_{\mathrm{N}}\left(\mathrm{Rb}^{+}\right)=4.0 \mathrm{~K}, T_{\mathrm{N}}\left(\mathrm{Cs}^{+}\right)=2.7 \mathrm{~K}). The internal hyperfine fields extrapolated to (T=0 \mathrm{~K}) are (\sim 51 \mathrm{~T}) suggesting zero point spin reduction ( (\sim 15 %) ) consistent with 1D behavior. High field d.c. susceptibility measurements for polycrystalline (\mathrm{Cs}{2} \mathrm{FeF}{5} \cdot \mathrm{H}{2} \mathrm{O}) sug. gest spin-flop behavior with (H{S F}<1 \mathrm{~T}). A qualitative comparison of the low-temperature magnetic properties of the (\mathrm{F}^{-})and (\mathrm{Cl}^{-}) analogues for the series (M_{2} \mathrm{FeX} X_{5} \mathrm{H}{2} \mathrm{O}\left(M=\mathrm{NH}{4}^{+}, \mathrm{K}^{+}, \mathrm{Rb}^{+}, \mathrm{Cs}^{+}\right)) is given. 1994 Academic Press. Inc.