An important problem in atmospheric chemistry is the determination of the oxidative capacity of the troposphere, which provides an index of how quickly trace gas species are destroyed. Central to this question are the abundance and distribution of hydroxyl radical, which is responsible for the destruction of a large proportion of chemical compounds such as CH4, CO, and nonmethane hydrocarbons. 14CO can be used as a chemical tracer to help constrain the OH concentration, as oxidation via OH is the major sink for all carbon monoxide. We present the first 14CO observations in the free troposphere during different seasons from 90¿S to 34¿N. Model simulations suggest that a large increase in the carbon monoxide sink strength may be needed in order to accommodate the experimental results. ¿ American Geophysical Union 1992