The reaction of 2,5-dimethylfuran and ethylene to produce p-xylene represents a potentially important route for the conversion of biomass to high-value organic chemicals. Current preparation methods suffer from low selectivity and produce a number of byproducts. Using modern separation and analytical techniques, the structure of many of the byproducts produced in this reaction when HY zeolite is employed as a catalyst has been identified. From these data, a detailed reaction network is proposed demonstrating that hydrolysis and electrophilic alkylation reactions compete with the desired Diels-Alder/dehydration sequence. This information will allow the rational identification of more selective catalysts and more selective reaction conditions.