The reaction cross sections of alkali metal atoms with bromine molecules have been obtained by direct measurements of alkali atom decay rates. The alkali atoms were produced in the presence of a known amount of bromine molecules by photodissociating the bromide of the particular alkali atom with a short pulse of uv light. As the alkali atoms reacted with bromine molecules their decay rate was monitored by observing the transmission of alkali atom resonance light through the vapor. The cross sections were computed from the decay rates. They are Na, 116 Å2; K, 151 Å2, Rb, 197 Å2, and Cs, 204 Å2. These results are accurate to about 15%. The results are compared with theoretical calculations.