The reactions between nitrogen dioxide (NO2) and conjugated dienes in the urban atmosphere may contribute to elevated atmospheric NO2 concentrations in winter pollution episodes. In response to a need for kinetic data for these reactions, a simple method for predicting rate constants for the reaction between nitrogen dioxide (NO2) and alkyl-substituted alkenes (monoalkenes and conjugated dienes) has been developed. A structure–activity relationship (SAR) based on the structure of the alkene is shown to be useful in providing an initial estimate of an unknown rate constant for such reactions. The SAR is formulated using rate constants predicted by a correlation between the logarithm of an experimentally determined rate constant and the calculated energy of the highest occupied molecular orbital (HOMO) of the alkene under consideration. The success of the approach is judged by the probability that the rate constant predicted by the SAR lies within a factor of two of the measured value for those monoalkenes and dienes for which experimental data exist. These probabilities were 82% for the dienes, but only 43% for the monoalkenes. However, rate constants for 94% of the dienes and 86% of the monoalkenes were predicted within a factor of three. For the dienes, at least, the SAR thus provides a good chance of providing a first estimate of the rate constant for cases where experiments have not yet been performed.