The microwave hydrothermal synthesis of silicalite zeolite was monitored in situ using small and wide angle X-ray scattering (SAXS and WAXS). A custom-built waveguide reactor provided the microwave heating at 2.45 GHz frequency. Microwave power up to 300 W was supplied by a SAIREM generator via a coax connection. A sample port in the waveguide section allowed a thin-walled glass sample vessel to be held in the microwave field. The sample vial was sealed with a fiber optic temperature probe used for monitoring the temperature. Silicalite zeolite precursor solutions were heated temperatures between 100 and 130°C for up to 120 minutes. The SAXS or WAXS patterns were simultaneously scanned during the reaction under microwaves via X-ray beam through slots in the side of the waveguide. The progress of silicalite zeolite formation was followed and the analysis of SAXS patterns showed the primary particles of 2 nm shifted to aggregates of 5.8 nm in diameter, in 120 minutes at 130°C.