One kind of magnetic solid acid (MSA) was prepared by thermolysis of nickel sulfate with ferric hydroxide at 1,123 K. Its properties, such as magnetism and surface acidity, were investigated by PPMS, FTIR-NH3, TPD-NH3, XRF, XRD techniques. Its saturation magnetization is estimated to be 10.72 emu/g and a hysteretic feature with a remnant magnetization (2.76 emu/g) and coercivity (0.15 kOe) at 300 K is exhibited. The results also reveal that MSA solely possesses mild Lewis acid sites and four crystalline phases. Its catalytic activity for esterification of ammonium lactate with n-butanol was tested. The esterification yield (Ey) will be significantly enhanced when SnCl2 center dot 2H(2)O is added. The highest Ey will be up to 87.3% when the complex concentration is 5 wt% (mass ratio of SnCl2 center dot 2H(2)O to MSA = 4:1). The possible reasons correlating with the esterification performance of the complex system are discussed.