Distributed feedback organic semiconductor lasers based on polyfluorene are shown to be suitable for use as chemical sensors for the detection of nitroaromatic-based explosive vapors. The laser threshold is increased by a factor of 1.8 and the slope efficiency is reduced by a factor of 3 after exposure to the vapor. The sensing efficiency depends strongly on the excitation energy with a maximum efficiency of 85%. The temporal dynamics of the laser response to the analyte have been investigated. The laser emission falls to 60% of its initial value in 46s. A model is developed to offer some insight into the diffusion of the vapor molecules inside the polymer layer.