Phase Doppler/laser Doppler anemometry (PDA/LDA) and microscopic high speed imaging have been applied to an air assisted spray using three different biodiesels (fatty acid methyl esters of short, medium and long chain length) and ethanol. The momentum decay and droplet size characteristics of the four fuels have been compared as a function of Reynolds number, mass loading, and radial position for a number of downstream locations. The PDA/LDA results suggest that the spray characteristics are very similar past x/D = 5 showing that the majority of break-up occurs in the near exit plane region, with minimal secondary atomization occurring further downstream. Microscopic high speed imaging has revealed qualitative information on the breakup structure as a function of physical properties and downstream locations showing significantly different atomization behaviour at the exit plane. An automated image processing technique has been applied to calculate the liquid blockage area as a measure of the spray density and degree of atomization. The technique has revealed the dependence of liquid blockage area on the fuel physical properties, showing that the long chain length biodiesel has more unbroken liquid at the exit plane. Furthermore, a manual processing method has been used to provide detailed statistical information on the probability of occurence of shapes such as long ligaments, short ligaments, unbroken liquid volumes, and deformed droplets appearing in the spray. The probability of a long ligament appearing in the long chain biodiesel is much higher while ethanol and the short chain biodiesel have yielded very similar results. In addition to revealing information on the atomization characteristics of these biodiesels, the two image processing techniques suggest a simple and alternative way of characterizing atomizing sprays.