We investigate the steady-state performance of a planar micromixer composed of several S-shaped units. Mixing efficiency is quantified by the decay of the scalar variance downstream the device for generic feeding conditions. We discuss how this decay is controlled by the spectral properties of the advection-diffusion Floquet operator, F, that maps a generic scalar profile at the inlet of a single unit into the corresponding profile at the unit outlet section. Two advantages characterize the Floquet operator approach -(i) it allows to analyze an arbitrarily long device and (ii) it provides a quantitative assessment of mixing efficiency that is independent of the feeding conditions and that depends solely on the interaction between advection and diffusion. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 318-335, 2010