A simple sampling and analytical method for the determination of OH radical concentrations in air is described, as applied to the characterization of air in closed environmental chambers. High-volume sampling through a filter paper impregnated with α-(4-pyridyl 1-oxide)-N-tert-butylnitrone leads to the formation of an OH-spin adduct. After extraction into toluene the adduct is analysed by electron spin resonance spectroscopy. Unambiquous identification and quantification of OH radical concentrations is achieved, giving a detection limit of 5 × 104 radicals cm−3 of air and a precision of ± 25%.