The rate constant for the reaction of Si+ with O2(a 1Δg) was measured in a selected ion flow tube (SIFT) between 200 and 500 K. The temperature dependence is T−(2.2±0.8). The rate constant at 500 K is below the detection limit (<1 × 10−11 cm3 molecule−1 s−1) while the rate constant at 200 K is 3.6 × 10−11 cm3 molecule−1 s−1. The product is SiO+, in accord with potential energy surfaces. These surfaces also show that the reaction of Si+ with O2(3Σg−) only yields association products, SiOO+ or OSiO+ ions. An upper mesosphere/lower thermosphere model predicts that reaction with O2(a 1Δg) is the most important removal process for Si+ during the day between 87 and 107 km, which may explain the depletion of this ion relative to other ions of meteoric origin, such as Fe+ and Mg+. The reaction therefore has an important influence on the composition and lifetimes of sporadic E layers during daytime.