AbstractFour questions relative to suction lysimeter performance have been raised by operators of land treatment systems. These questions deal with plugging of the porous segments of the lysimeters, soil suction operational ranges, adsorption onto and screening by the materials comprising the various lysimeter parts, and loss of volatile organics under negative pressures. This article first describes the physical operation of a lysimeter, the characteristics of the lysimeters tested (high‐ and low‐flow lysimeters with porous ceramic cups and lysimeters with porous PTFE (polytetrafluo‐roethylene) cups, and procedures followed in preparing them for testing. The results of experiments dealing with the first two questions plugging and operational ranges are then discussed. It was found that the intake rate of suction lysimeters placed in the vadose zone, in most types of soils, will initially drop off rapidly, but will stabilize after about 15 liters of moisture have been drawn through the porous cups. Packing a crystalline silica flour slurry around the cups of PTFE lysimeters negates most of the plugging associated with finer particles in soils. The effective operating range of ceramic lysimeters is between 0 and 60 centibars of suction independent of the use of silica flour. The operating range of PTFE lysimeters without silica flour is extremely narrow, but with the use of silica flour is extended to about 7 centibars of suction. The testing program indicates that all lysimeters should be checked for leaks using pressure techniques before field installation and that lysimeters have “dead” spaces, or reservoirs of from 34 to 80 mL of moisture that cannot be extracted from the cups, which must be taken into account when determining moisture collection rates. Information gathered in the test program is planned for inclusion in an EPA guidance document entitled “Unsaturated Zone Monitoring at Hazardous Waste Land Treatment Units.”