An analysis is made of 13 years of observations of ozone concentrations in the remote marine boundary layer at Cape Grim, Tasmania 41 ¿S. These data reveal a decrease in ozone concentration in the first few hours following sunrise at a rate of around 0.1 ppb h-1 in mid-summer and in mid-winter. This ozone destruction phenomenon causes an asymmetry in the daily ozone loss rate with enhanced destruction following sunrise, is statistically distinguishable from the O3-HOx destruction cycle that peaks at mid-day, and occurs at similar rates in mid-summer and in mid-winter. The cause of this sunrise ozone decrease is examined using the conservation equation for ozone. We speculate that ozone destruction at sunrise arises due to halogen chemistry. The absence of sunrise ozone decrease in models of marine boundary-layer photochemistry means that the ozone destruction rate in the remote marine boundary layer is underestimated by perhaps a factor of two. ¿ 2000 American Geophysical Union