In order to enable a realistic assessment of the 131I radiation exposure of the population living in the vicinity of light water reactors (LWRs) in normal operation, measurements were taken of the fractions of physicochemical 131I species in the stack exhaust air of five LWRs in the Federal Republic of Germany over periods of up to 3 yr. It is concluded that for these LWRs generally conservative values of the calculated 131I radiation exposure result if fractions for the radioecologically critical elemental131I and for the far less important organic 131I of 50% each are used.