We present a simple method for fabricating and operating normally open, electrothermally actuated microvalves. These valves are fabricated by placing a gas-permeable elastomeric membrane between two etched glass plates. The reservoirs and channels on one layer are filled with a low melting point polymer (polyethylene glycol, PEG) that exhibits a large volumetric change (of up to 30%) upon phase transition (melting). This volume expansion is used to actuate the membrane and seal the microfluidic channels located in the second etched glass plate. The PEG in the reservoir is heated with integrated patterned platinum-resistive elements. The valve reliably seals the microfluidic channel against external fluid pressures of 10 psi. This valve can be readily integrated with one of the standard technologies for lab-on-a-chip (LOC) fabrication and is suitable for use with the polymerase chain reaction. The novelty of this microvalve lies in the ability to fill dead-end microchannels with a polymer, its self-sealing ability, the ability to remotely actuate the valve by transferring pressure via a microchannel and the compatibility of this microvalve with standard LOC technologies.