An apparatus was constructed to study pesticide degradation in situ under vadose‐zone conditions. The apparatus consists of a 15‐cm (i.d.) casing and an enclosed vadose exposure chamber wherein pesticide‐treated soil cores, placed in the chamber, can be equilibrated with the surrounding vadose zone at selected depths. Monitored for 100 d, O 2 concentrations at 0.5‐, 1.0‐, and 2.0‐m depths were nearly identical to O 2 concentrations in control samplers, indicating that the apparatus was in equilibrium with the soil atmosphere and is appropriately isolated from the surface atmosphere. Both radiolabeled and unlabeled pesticides can be studied in the apparatus because the chamber is self‐sealing in the event of unexpected flooding.