Hydrophilization of poly(methyl methacrylate) (PMMA) substrates has been demonstrated using a Xe*(2) excimer lamp radiating vacuum ultraviolet (VLJV) light of 172 nm in wavelength. In this study, we have particularly focused on the effects of atmospheric pressure during VUV irradiation. Each of the substrates was photoirradiated with VUV light under a pressure of 10, 10(3), or 10(5) Pa. Although in each case the hydrophobic PMMA surface became hydrophilic, the water-contact angle and photooxidation rate markedly depended on the atmospheric pressure. The sample treated at 10 Pa was less wettable than the samples treated at 10(3) and 10(5) Pa due to the shortage. of oxygen molecules in the atmosphere. The minimum water-contact angles of the samples treated at 10, 10(3), and 10(5) Pa were about 40, 25, and 24degrees, respectively. Microfabrication of the PMMA substrates was Also demonstrated employing a simple mesh-mask contact method using the same excimer lamp. As confirmed by atomic force microscopy, a photoetched groove composed of 25 x 25 mum(2) features was successfully fabricated on the PMMA substrates. Both the spatial resolution and photoetch depth of the microstructures depended on the atmospheric pressure. At 10 and 103 Pa, we achieved finely grooved microstructures at etching rates of 13 and 13.2 nm/min, respectively. In comparison, when the pressure was further increased to 10(5) Pa, the etching rate decreased to 6.9 mn/min and patterning resolution became significantly worse. The pressure of 10(3) Pa was determined to be optimum for accurately defining PMMA surfaces both chemically and geometrically.