In this study, we provide a simple and reproducible method for the preparation of highly active and recyclable colloidal acid catalysts. First, 16-heptadecenoic acid-functionalized magnetite nanoparticles were encapsulated in monodisperse cross-linked polymer spheres. This was achieved by emulsion copolymerization technique in an aqueous phase of styrene and divinylbenzene (DVB). Different ratios of styrene and DVB were used to tune the structural stability and surface morphology of the composites. With increase in DVB content, the surfaces of the colloidal composites become increasingly rougher. The obtained colloids were functionalized with sulfonic acid groups to obtain magnetically recyclable catalysts with H+ contents in the range of 2.2−2.5 mmol g−1 and surface areas of 45−120 m2 g−1. For the condensation reaction of benzaldehyde and ethylene glycol, magnetic acid catalyst prepared only from DVB precursor was found to be active and with high selectivity and long-term stability.