We report the first experimentally-determined metal isotope equilibrium fractionation factors for a metal sulphide at ambient temperatures and pressures. Mackinawite, referred here as FeS_m (where the subscript m indicates mackinawite), can be a reactive component in diagenetic pyrite formation and the extent of equilibration between FeS_m and dissolved Fe(II) has direct implications the δ⁵⁶Fe signatures recorded in diagenetic pyrite. The measured equilibrium Fe isotope fractionation between Fe(II)_aq and FeS_m is Δ⁵⁶Fe_Fe(II)-FeS=-0.52±0.16‰ at 2°C and Δ⁵⁶Fe_Fe(II)-FeS=-0.33±0.12‰ at 25°C and pH 4. At the experimental pH the equilibrium fractionation factor between all dissolved Fe(II) species and FeS_m (Δ⁵⁶Fe_Fe(II)-FeS) equates to the fractionation factor between Feaq2+ and FeS_m (Δ56FeFe2+-FeS). The measured fractionations are of the same order as other non-redox fractionations measured in low-temperature Fe-C-O systems. We show that at low temperature, the Fe(II)_aq-FeS_m system is slowly asymptotic to isotopic equilibrium and consequently, FeS_m is likely to partially conserve kinetically derived isotopic signatures generated on precipitation. Combined with the range of published kinetic fractionations measured on FeS_m precipitation, our data suggest that, subject to the degree of isotope exchange during equilibration, FeS_m can display δ⁵⁶Fe compositions encompassing a range of ~1.4‰.