In this paper a new method is presented for the determination of the skin thickness of asymmetric ultrafiltration membranes. The method is based on the use of well-defined, uniformly sized colloidal gold particles, permeated from the sublayer side of the membrane, combined with electron microscopic analysis of the membrane afterward. Using this method poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and polysulfone (PSf) membranes were investigated. PPO membranes appeared to have a well-defined skin layer with a thickness of about 0.2 μm and a pore size distinctly different from that in the macroporous layer underneath. In the case of PSf such a distinct skin layer cannot be defined. The size of the pores in these membranes gradually increases from skin to sublayer.