A trial was conducted in the Huon Valley of Southern Tasmania in the 1986–87 season, where 26-year-old red ‘Delicious’ apple trees were thinned with ethephon sprays 1 or 2 days after full bloom. Application rates of 250–1500 g a.i. ethephon/ha were used. These were applied in a concentration range of 35–750 ppm and with spray volumes of 1000, 2000, 4000, 6000, and 7000 litre/ha using a conventional, hydraulic, air-blast sprayer or a hand lance. The volume of spray used was of critical importance in the ethephon thinning of mature red ‘Delicious’ trees. The results showed that at least 4000 litre/ha was necessary to effectively thin these trees and the effect was not closely allied to the concentration or dosage used. The majority of concentrations lower than 100 ppm proved quite effective as thinners when sprayed at higher volumes. Thinning rates were correlated with volumes rather than concentration or dosage of spray used. This was expressed in the form of a linear, negative regression between fruit thinning or a significant positive regression between mean fruit weight and spray volume. This emphasised the need to increase the volume of spray used to thin red ‘Delicious’ with ethephon rather than increase the concentration. Thinning with ethephon was highly dependent on the volume rather than on concentration or dosage of spray.