To study the homogeneous nucleation of thiourea adducts, the metastable zone widths for the thiourea adducts formed with guests of cyclohexane, methylcyclopentane (MCP) and their mixtures were determined. The influence of moderate agitation rate and saturation temperature on the maximum allowable undercooling (i.e., metastable zone width) was studied. The metastable zone width is markedly affected by the cooling rate and the agitation rate. MCP in guests roles to retard the nucleation rate of cyclohexane-MCP-thiourea adducts by causing k(2) to decrease. The order of the nucleation in the nucleation process of thiourea adducts is in the range between 3 and 6, and appears to be dependent on the saturation temperature and the guest concentration. The nucleation rate constant (i.e., frequency factor (k(2)/N-obs)) has shown to exclusively depend on the concentration of guest and the temperature. The retarding effect of MCP in the thiourea adduction was found to be caused by, not only the increase of surface energy, but the decrease of k(2)/N-obs.