An investigation has been carried out concerning the transmission of thermal and fast neutrons in air-filled annular ducts. A calculational model to predict the air-gap flux is developed and fitted to the results from foil-activation experiments in a Fe-D2 O configuration, the duct length being about half a meter and the annular air gaps varying from 0.5 to 2.0 cm. It is based upon the condition that the flux is theoretically and experimentally divided into uncollided and collided components for both thermal and fast neutrons. The model may be applied to most of the straight annular ducts that occur in reactor shielding, but it is especially designed for the problems met with in short ducts.