A method for the evaluation of composition of a bitumen-air aggregate floating in a separation vessel to the froth layer is presented. The mass of bitumen contained in the bitumen-air aggregate was determined on the basis of measured aggregate size and rise velocity inside the separation vessel. Relationships describing the velocity of unloaded, contaminated and uncontaminated bubbles were used as a ＇reference state＇ that was needed to evaluate the composition of the bitumen-air aggregate, They were used to illustrate how a choice of the ＇reference state＇ can affect the calculated mass of bitumen in an aggregate. It was found that the mass of bitumen contained in a bitumen-air aggregate was in the range of 10(-6) to 10(-4) g depending on the aggregate dimension and velocity relationship used. The evaluated equivalent diameter of the aggregates was within the range of 0.2-1.2 mm. The approach presented here can be generalized to other gas-solids flotation systems.