Membrane filtration is the preferred method for sterility testing. Viscous products, such as oils, creams and ointments can prove difficult to filter. As per USP <71> viscous oils may be diluted as necessary with a suitable sterile diluent such as isopropyl myristate shown not to have antimicrobial activity in the conditions of the test. Sterile isopropyl myristate (IPM), has been shown to have no anti-microbial activity in test conditions. This study investigates the effectiveness of IPM as a diluent by comparing filtration times with unfiltered oils.