Elastomeric microvalves in poly(dimethylsiloxane) (PDMS) devices are today's paradigm for massively parallel microfluidic operations. Here, we report that such valves can act as nanochannels upon closure. When tuning nanospace heights between ~55 nm and ~7 nm, the nanofluidic phenomenon of concentration polarization could be induced. A wide range of concentration polarization regimes (anodic and cathodic analyte focusing and stacking) was achieved simply by valve pressure actuation. Electro-osmotic flow generated a counterpressure which also could be used to actuate between concentration polarization regimes. 1000-fold preconcentration of fluorescein was achieved in just 100 s in the anodic focusing regime. After valve opening, a concentrated sample plug could be transported through the valve, though at the cost of some defocusing. Reversible nanochannels open new avenues for integrating electrokinetic operations and assays in large scale integrated microfluidics.