A process for producing a micromachined tube (microtube) suitable for microfluidic devices. The process entails isotropically etching a surface of a first substrate to define therein a channel having an arcuate cross-sectional profile, and forming a substrate structure by bonding the first substrate to a second substrate so that the second substrate overlies and encloses the channel to define a passage having a cross-sectional profile of which at least half is arcuate. The substrate structure can optionally then be thinned to define a microtube and walls thereof that surround the passage.