To characterize differences in soybean resistance to salt stress, two soybean species, the wild salt-tolerant soybean Glycine cyrtoloba (serial number ACC547) and the cultivated salt-sensitive soybean G. max (cv. Melrose) were treated with 0, 50, 100, or 150 mmol L⁻¹ NaCl for 5 days. A series of physiological parameters were determined in both shoots and roots, including content of chlorophyll (Chl) and malondialdehyde (MDA); electrolyte leakage (EL); hydrogen peroxide (H₂O₂) concentration; superoxide oxygen radical (O₂⁻) production rate; activities of several enzymes including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD); and selective ion (Na⁺ and K⁺) accumulation. Our results showed that the relative salt tolerance of ACC547 was associated with lower loss of Chl content; lower MDA content, EL, H₂O₂ concentration, and O₂⁻ production rate in both shoots and roots; higher POD activity caused by new isoforms in roots; and higher K⁺ concentration and K⁺/Na⁺ ratio in shoots. These results suggested that relative lower membrane injury, efficient K⁺ vs. Na⁺ selective accumulation, and newly induced POD isoenzymes are mechanisms of salt tolerance in soybean.