Thick thermally isolated microheaters in Si are fabricated using high aspect ratio etching technology. These thick microheaters a with large surface area provide the large adsorbent capacity needed for high sensitivity microfabricated preconcentrators in a microgas chromatography system. In addition, air gaps around the microheaters provide good thermal isolation to lower the power consumption. A 520 mum thick Si microheater with good thermal isolation has been made and its back side is anodically bonded to a pyrex glass substrate. To provide good thermal isolation, a 500 mum wide air gap around-the microheater, thin poly-Si interconnects on top of dielectric membrane, and air gap isolation from the bottom glass substrate are used. Microheaters with a 500 mum air gap can be heated up 50% faster to a temperature of 270 degreesC compared to those with a 100 mum air gap, while the power consumption is, 25% less and there is a larger temperature difference between the microheater and the bonding area. Operating the microheater in a vacuum results in lower power consumption. At 250 degreesC, 35% less power is needed at 1.2 Torr compared to atmospheric pressure. The power consumption is further reduced by minimizing the contact area with the heater support substrate,. Up to 34% power reduction has been demonstrated by placing heaters on a thin membrane or etching trenches in the supporting substrate. These thick thermally isolated Si microheaters can provide good power efficiency, large adsorbent capacity, and high mechanical strength for microfabricated preconcentrators. (C) 2003 American Vacuum Society.