A miniaturized approach was developed for quantitative permethylation of oligosaccharides, which involves packing of sodium hydroxide powder in microspin columns or fused-silica capillaries (500 μm i.d.), permitting effective derivatization in less than a minute at microscale. Prior to mass spectrometry, analytes are mixed with methyl iodide in dimethyl sulfoxide solution containing traces of water before infusing through the microreactors. This procedure minimizes oxidative degradation and peeling reactions and avoids the need of excessive clean-up. Picomole amounts of linear and branched, sialylated and neutral glycan samples were rapidly and efficiently permethylated by this approach and analyzed by matrix-assisted laser desorption/ionization mass spectrometry.