This research used montmorillonite (K10) modified with Hexadecyltrimethylammonium bromide (HDTMABr), and sulfuric acid (H2SO4). The samples are marked with MMT, OMMT for organic-modified montmorillonite, and AMMT for inorganic-modified montmorillonite. UF resin with a molar ratio FA/U = 0.8 was synthesized in situ with modified and unmodified MMT. X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), non-isothermal thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were used to characterize the MMT samples. The degree of activation was determined based on the measurement of specific surface area, which was determined by the Sears method. The sulfite method was used to determine free and released formaldehyde from synthesized urea-formaldehyde/montmorillonite (UF/MMT) composites. SEM analysis showed changes in the OMMT morphology and the formation of a hollow network, affecting the clay's absorption capacity. Measurement of the specific surface area shows that higher values were obtained for AMMT (183 m2/g) compared to OMMT (13.5 m2/g). Despite that, the free and released formaldehyde amount was 0.06% and 4.6% for UF/AMMT and 0.1% and 1.0% for UF/OMMT. The larger interlayer spacing and hydrophobic nature of OMMT make it an effective barrier within the UF resin matrix.