The consequences on the chemistry in diffuse clouds of the injection of small hydrocarbon molecules is explored. It is assumed that the injection arises from the erosion of carbonaceous grains at cloud boundaries, and that the injected species take part in conventional interstellar chemistry. The models indicate that for plausible injection rates the “daughter” species of the injected “parent” molecules should appear in significant amounts, at least during the injection period, and that daughter species can appear in gas that is otherwise almost entirely atomic. Therefore, injection of hydrocarbons is a mechanism that is capable of providing polyatomic molecules in detectable amounts in diffuse atomic regions of the interstellar medium. The implications of this result for the carriers of the unidentified diffuse interstellar bands are briefly described.