This paper presents formulae that we have derived recently for the rate constants of some chemical reactions important in interstellar chemistry. First of all, the application of a simple classical capture theory to the rate constants for the fast reactions of ground‐state carbon atoms with organic molecules is described. It is found that the rate constants scale with the size of the molecule as N1/3, where N is the number of carbon atoms in the molecule. This is found to be in remarkable agreement with experiental results for the reactions of alkenes and alkynes, even up to N=16. Rate constants scale with the size of the molecule as N1/3, where N is the number of carbon atoms in the molecule. This found to be in remarkable agreement with the experimental results for the reactions of alkenes and alkynes, even up to N=16. Rate constant formulae at very low temperatures for the fast reactions of of molecules in Σ and Π electronic states with various ionic and molecular partners, including those in 2Π states...