The mutualistic association between plants and arbuscular mycorrhizal fungi (AMF) often leads to improved nutritional status of the host plant (see Koide and Schreiner 1992). The contribution of mycorrhizal hyphae to acquisition and transport of nutrients to plants is difficult to determine experimentally because of nutrient transport by other processes in the soil such as solution diffusion and mass flow (Johansen et al. 1992; Frey and Schüepp 1993). The use of hydrophobic polytetrafluoroethylene (PTFE) membranes was suggested for nutrient transport studies in mycorrhizal research (Mäder et al. 1993) because this type of membrane is permeable to mycorrhizal hyphae but does not permit mass flow and diffusion of ions. The membrane consists of a microporous, symmetrically structured PTFE layer coated onto a polyester tissue and is reported to be hydrophobic (Schneider and van Gassel 1984). Here we present the results of three experiments to estimate the efficacy of PTFE membranes in preventing diffusion and mass flow of ions. Two experiments to test the transport of ¹⁵N across a PTFE membrane using plants and AMF were conducted in two laboratories independently of each other, while the third experiment tested the characteristics of two different PTFE membranes and was conducted in an abiotic experimental system using Cu solutes.