An oil shale formation may be treated using an in situ thermal process. Heat may be provided to the treatment area. Heat may be allowed to transfer from at least one heat source to a section of the formation. Pressure within a portion of the formation may be controlled. Hydrocarbons, H₂, and/or other formation fluids may be produced from the formation. In some embodiments, the pressure may be controlled to obtain a desired property in the produced fluids.