Halocarbon concentrations were obtained from the upper troposphere and lower stratosphere over Alaska in May 1975 using aircraft sampling techniques and subsequent laboratory analysis. The data demonstrate two phenomena: (1) a rapid decrease in the halocarbon concentrations across the tropopause into the lower stratosphere, and (2) variable day-to-day concentration gradients either decreasing or increasing upward from the tropopause depending upon meteorological conditions. An air mass transport mechanism through the tropopause into the stratosphere is proposed to explain the temporal and spatial variations observed.