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Microwave-assisted hydrothermal synthesis is a simple, reproducible, rapid, and effective method, and therefore, has attracted considerable interest among scientists in the field of synthesis not only of iron oxide but also of other metal oxides. This method has been used for the synthesis of iron oxide nanoparticles, hematite (alpha-Fe2O3), and goethite (alpha-FeOOH) in strongly alkaline media with iron(III) chloride (FeCl3) as precursor under microwave emission of 850 W. In this work, the effect on the size, shape, and composition of the final product was investigated by changing the conditions, such as shortening the synthesis time, increasing the synthesis temperature, and adding various substances to the synthesis pathway. Samples synthesized at 200 degrees C for 20 min by increasing the added percentage of polyethylene glycol (PEG), sodium dodecyl sulfate (SDS), rosemary essential oil and lavender essential oil promote goethite-to-hematite transformation, while N-guanylurea sulfate (NGS) and sage essential oil behave differently. The aforementioned substances added at 260 degrees C resulted in a decrease in particle size, but pure hematite was obtained, regardless of the type of the substances added or the decrease in synthesis time from 20 min to 5 min. Furthermore, the use of essential oils rather than surfactants in this study was presented as a novel, environmentally friendly method of iron oxide synthesis.