The mixing due to helical flows in curved micro channels is investigated. A new chaotic mixing mechanism is presented relying on alternately switching between different flow patterns exhibiting four Dean vortices. Flow patterns and interfacial stretching factors are numerically computed for various Dean numbers. For experimental studies a prototype of a chaotic mixer with curved channels was fabricated. The experimental evaluation of the mixing performance corroborates the numerical prediction: the mixing performance found for Dean numbers above 140 is qualitatively different from that at lower Dean numbers; the periodic switching between different vortex patterns leads to efficient mixing, manifesting itself in an exponential growth of interfacial area. In addition to the studies on mixing, residence-time distributions in the mixing channel are computed numerically. These investigations indicate that due to mass-transfer enhancement originating from the transversal redistribution of matter in the chaotic flow, hydrodynamic dispersion is substantially reduced relative to a straight channel. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2297–2305, 2004