The properties of compounds of zinc stearate (ZnSt) and the zinc salt of a lightly sulfonated atactic polystyrene ionomer are described. ZnSt was miscible with the ionomer glass at concentration <5 wt. %. The presence of molten ZnSt significantly lowered the terminal relaxation time and the melt viscosity of the ionomer, which greatly improved its melt processability. The effects of the ZnSt addition on the mechanical properties of the ionomer were uncertain. The addition of ZnSt lowered the tensile modulus of the ionomer but improved the tensile strength for compounds with <10 wt. % ZnSt. The addition of ZnSt embrittled the compound due to the phase separation of crystalline ZnSt. However, the general conclusion of the effect of ZnSt on the mechanical properties was clouded by the fact that less than half of ZnSt crystallized due to the very slow crystallization kinetics of the fatty acid in the compound. (C) 2018 The Society of Rheology.