We attempted to wash liquid paraffin trapped in a porous polyethylene film by microemulsions formed in a polyoxyethylene-type nonionic surfactant system. Detergent efficiency reached the highest value, approximately 90-95%, in the case of the high oil-to-water ratio in microemulsions, but failed to reach the highest level when the oil content in microemulsions was low. Washing time required to reach the highest detergent efficiency was shorter with a short-chain surfactant than a long chain one, but the highest detergent efficiency was higher with the long-chainsurfactant, about 90% in the C10EO4 system and 95% in the C(16)Eo(7) system. The detergent efficiency with a mixture of long- and short-chain surfactants was lower than that with a long-chain surfactant alone. The high detergent efficiency with the microemulsions of high oil content is thought to be due to low interfacial tensions at the liquid paraffin-microemulsion and polyethylene-microemulsion interfaces; and the short washing time with short-chain surfactant is thought ot be due to the small size of micro domain inside the microemulsions.