Mesoporous monetite (CaHPO4), obtained by mechanochemical treatment of previously synthesized brushite (CaHPO4·2H2O), was used as efficient adsorbent for the organic pesticide malathion. The structure of brushite was confirmed by Raman spectroscopy. The phase transformation process was investigated by X-ray powder diffraction (XRD) and Fourier transformation infra-red spectroscopy (FTIR). The microstructure and morphology were determined by scanning electron microscopy (SEM) and the nitrogen adsorption–desorption method. It was found that five minutes of milling induces brushite–monetite phase transformation. Adsorption of malathion from aqueous solutions showed that this pesticide can be successfully adsorbed on surface of this material.