The aviation fuel market is one of the few areas in the petroleum industry where an increase in future demand can be predicted with a reasonable degree of certainty. However, this growth in demand is likely to be influenced by two major constraints, price and availability. At present aviation fuel represents only a relatively small portion of the barrel, but any increase in this fraction is limited by the need to meet specification requirements and competition from other products. The changing pattern in product demand is focusing attention on secondary refinery processes to maximise output of distillate fractions and on the use of alternative non-petroleum sources of fuel. In both of these approaches, significant upgrading of the initial product, involving both large capital outlay and high operating costs, may be necessary to produce final products within current specification limits. Under these conditions it is desirable to identify ways of easing specification requirements that will increase flexibility, but at the same time maintain product quality.