The physical and chemical characteristics of fuel can affect its flow, atomization, and combustion, all of which help to define the overall performance of a heating system. The objective of this study was to evaluate the effects of some important parameters of fuel quality on the operation of oil-fired residential heating equipment. The primary focus was on evaluating the effects of the fuel's sulfur content, aromatics content, and viscosity. Since the characteristics of heating fuel are generally defined in terms of standards (such as ASTM, or state and local fuel-quality requirements), the adequacy and limitations of such specifications also will be discussed. Liquid fuels are complex and their properties cannot generally be varied without affecting other properties. To the extent possible, test fuels were specially blended to meet the requirements of the ASTM limits but, at the same time, significant changes were made to the fuels to isolate and vary the selected parameters over broad ranges.