Two novel and environmentally benign organosulfonic acid-functionalized silica-coated magnetic nanoparticle catalysts 1 (Fe3O4@SiO2@Et-PhSO3H) and 2 (Fe3O4@SiO2@Et-PhSO3H) were prepared and their surface hydrophobicity was assessed by esterification reaction of fatty alcohols. Also, their water-toleration in two water-generating reactions was investigated, namely: synthesis of bis(indolyl)methane and the Biginelli reaction of benzaldehyde, methyl acetoacetate, and urea under solvent-free conditions. In these reactions, the catalyst 2 that was more hydrophobic and water-resistant showed higher catalytic activity and was characterized extensively by various techniques. This catalyst was successfully used in the Biginelli reaction of a series of aldehydes, beta-keto esters, and urea/thiourea for the synthesis of a diverse range of pharmacologically active 3,4-dihydropyrimidin-2-one/thione (DHPM) derivatives. The catalyst 2 was easily separated by an external magnet and the recovered catalyst was reused in five cycles without any significant loss of activity. (C) 2013 Elsevier B.V. All rights reserved.