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A reaction rate associated with the nighttime formation of an important diurnally varying species, N₂O₅, is determined from MIPAS-ENVISAT. During the day, photolysis of N₂O₅ in the stratosphere contributes to nitrogen-catalysed ozone destruction. However, at night concentrations of N₂O₅ increase, temporarily sequestering reactive NO_x NO and NO₂ in a natural cycle which regulates the majority of stratospheric ozone. In this paper, the reaction rate controlling the formation of N₂O₅ is determined from this instrument for the first time. The observed reaction rate is compared to the currently accepted rate determined from laboratory measurements. Good agreement is obtained between the observed and accepted experimental reaction rates within the error bars.