Freestanding, high aspect ratio microstructures in Si were micromachined as thick microheaters. These microheaters, combined with adsorbents, can be used as preconcentrators in a micromachined gas chromatography system. Dry etching of Si using etch and passivation cycles has been developed to produce 240 mum thick Si microheaters with 3 mum wide wires, achieving a high aspect ratio of 80:1. This optimized dry etching, technology results in high etch rates with vertical profiles for thick Si microheaters up to 535 mum. Microheaters with 40 mum wide wires, 110 mum gaps, 400 mum thick, and an area of 9 mm 2 have been fabricated. With the heater on a 140 mum thick Si membrane, it takes 1320 mW to increase the temperature by 290 degreesC. The power consumption is reduced to 447 mW for the same temperature raise with a freestanding Si microheater. Heating response for freestanding Si microheaters with different thicknesses is also studied. These Si microheaters have fast response times and reach 75% of the final temperature in 5 s. Microheaters consisting of Si wires and posts show similar response time and heating uniformity. In addition, power consumption is reduced by thermal isolation of the microheaters. These thick, freestanding high aspect ratio Si microheaters can provide high power efficiency, large adsorbent capacity, and high mechanical strength.