This paper explains the procedure used to determine the amount of air leakage being generated from the use of a drop-through rotary valve in a full-scale pneumatic conveying system. When producing a set ofpneumatic conveying characteristics for a test program, rotary valve air leakage is often not considered. In reality, the air mass flowrate in the conveying line will be less than the total supplied air mass flowrate due to air leakage through the rotary valve. By measuring the air leakage and adjusting the total supplied air mass flowrate to represent the actual air mass flowrate travelling through the conveying line, a more accurate representation of the conveying conditions can be obtained. Hence the design of pneumatic conveying systems can be determined more precisely. Tests have been performed and plots have been produced showing both the pneumatic conveying characteristics before and after rotary valve air leakage has been taken into account. These results show that rotary valve air leakage can have quite an effect, with some tests losing more than 50010 ofthe supplied air mass flowrate through the rotary valve.