A Taylor-Aris dispersion in laminar capillary flows of magnetic nanofluids submitted to transverse rotating magnetic fields (RMFs) was analyzed with a simple phenomenological mixing approach. The nanofluid residence time distributions (RTDs) measured under RMFs were used to quantify the deviations, with respect to field-free Poiseuille flows, of the axial dispersion induced by the rotating magnetic nanoparticles (MNPs) as a function of MNPs concentration and diameter, and RMF frequency and strength. The attenuation of axial dispersion due to the magnetic field was ascribed to an enhanced transverse diffusion coefficient that thrust tracer radial transport, owing to nanoconvective streams in the nanoparticle neighborhoods, beyond molecular diffusion capability. To estimate the enhanced transverse diffusion coefficient, a semiempirical model was developed in which the nanofluid domain was viewed as an array of identical cells each containing a magnetic nanoparticle at its center. Owing to the nanoparticle rotation in the magnetic field, each cell consisted of an inscribed perfectly mixed core confined in a stagnant shell where molecular diffusion prevailed. Two- and three-dimensional diffusion simulations of the two-zone cell were used to quantify, and link, the size of the mixed core to the measured axial dispersion coefficients under various experimental conditions.