The hydrothermal crystallization of sodium zeolite A from amorphous aluminosilicate gels at 80-120 degreesC has been studied by time-resolved in situ energy-dispersive X-ray diffraction. This has allowed the course of crystallization to be followed in greater detail than previously possible. Quantitative crystallization curves have been obtained, and these kinetic data are analyzed using the Avrami-Erofe'ev nucleation growth model to allow a simple means of determining the crystallization rate. We observe that the choice of silica starting material can affect the course of reaction; the use of fumed silica gives rise to a distinctive two-stage growth curve. This unusual crystallization behavior is also dependent on both NaOH concentration and temperature. At the highest NaOH concentrations and when the amount of water is low, zeolite A is only present for a short time and hydroxosodalite is the sole product on continued heating. We discuss our new in situ observations in relation to previous quenching studies of zeolite crystallizations, and their implications in understanding zeolite formation mechanism.