Two bifunctional compounds, 12-azido-1-diazo-2-dodecanone (A) and 1-diazo-17-octadecene-2-one (B), show an ability to act as coupling agents in fiberglass/ polyethylene composites. Under appropriate conditions the diazoketone functional groups in both A and B react with hydroxyl groups on a fiberglass surface, whereas the azide group in A and the alkene group in B form bonds with the plastic matrix during processing. FTIR and NMR spectroscopy were used to study the decomposition of each of these compounds under heat and UV light. Each treatment resulted in a relatively fast decomposition of the diazoketone functional group, along with a slower reaction of the azide and alkene groups. Thus it was possible to react the diazoketone end of these compounds with a fiberglass surface, without affecting the azide or alkene functional groups on the other ends of the molecules. In samples of treated fiberglass containing compounds A or B and mixed by extrusion with polyethylene, the mechanical properties of the composites had improved properties over composites containing untreated samples of fiberglass. With A as the coupling agent, both the tensile properties and Izod impact showed changes that indicated that a bifunctional bridge was formed between the fiberglass and polyethylene phases.