In order to understand the crystallization conditions of zeolite A, induction periods for nucleation from clear aluminosilicate solutions were measured and correlated with operating variables. The induction periods were found to decrease with supersaturation of reactant solutions and were represented by two lines when they were plotted against the supersaturation on a semilog ordinate system at given temperature and given NaOH concentration. At higher supersaturations, amorphous phases precipitated, while zeolite A crystals were obtained at lower supersaturations. For precipitation of zeolite A, the induction periods decreased with the NaOH concentrations, indicating that the crystallization rate of zeolite A was accelerated by the presence of NaOH. The boundary between the precipitation of zeolite A and that of amorphous phases was found to depend on the NaOH concentration and the ratio of [Al3+] to [NaOH] as well as saturation ratios. Finally, an attempt was made to determine the activation energy for nucleation and the surface energy of zeolite A on the basis of measured induction periods.