An inkjet assembly comprising a vented ink reservoir for containing a liquid ink therein, the vented ink reservoir defining an internal volume occupied at least in part by a semipermeable membrane in fluid communication with a vent that automatically adjusts for pressure differentials by enabling gaseous diffusion between an environment external to the vented ink reservoir and the internal volume of the vented ink reservoir, while inhibiting liquid diffusion therethrough. A method is also disclosed for mounting the semipermeable membrane to at least one of an ink reservoir cap and an ink tank.