The suitability of using octafluorocyclobutane (C 4 F 8 ) patches as hydrophobic valves in microfluidic biochemical applications has been shown. A technique has been developed to generate lithographically defined C 4 F 8 hydrophobic patches in deep reactive ion-etched silicon channels. Some of the advantages of this process are that no specific cleaning of the substrate is required, C 4 F 8 is deposited on the sidewalls and the bottom of the channels, a standard photoresist mask can be used to define the patches, and that it is a fast and convenient dry chemical process performed by a standard inductively coupled plasma etcher using the Bosch process.Different patch lengths (200-1000μm) of C 4 F 8 were deposited in 50μm wide channels to evaluate which size is most suitable for microfluidic biochemical applications. The valve function of the hydrophobic patches was tested for the following liquids: DD water, acetone, propanol, bead solution and a mixture used for pyrosequencing of DNA. Patch lengths of 200μm of C 4 F 8 successfully stopped each solution for at least 20 consecutive times. The C 4 F 8 film resists water for at least 5h. The hydrophobic valve also resists very high concentrations (25%) of surfactants (Tween 80). C 4 F 8 shows a much higher resistance towards water and surface active solutions than previous hydrophobic patches. However, 50% Tween 80 was not stopped at all by the hydrophobic patch. An applied pressure of 760Pa at the inlet was needed for water to over-run the hydrophobic patch.