A petroleum refinery plant in Taiwan generates approximately 2.8 x 10(4) tons of oily sludge, including the residual from storage tanks, and the scum sludge and waste-activated sludge from the wastewater treatment plant. Experimental results indicate that freezing and thawing can transform the flocs into large and compact aggregates and significantly enhance the sludge deliquorability for scum and waste-activated sludges. The results are attributed to the advance of an ice front during freezing that expels oil phases from the sludge body. Cationic polyelectrolyte conditioning only slightly influences sludge with a high oil content because the chemical conditioner largely binds to oily droplets on the sludge flocs. However, neither freeze/thaw treatment nor polyelectrolyte conditioning, can satisfactorily condition the tank residual. Underlying mechanisms of oily sludge conditioning are discussed.