A bubbler chamber assembly comprising one chamber or two or more chambers connected in series all chambers being in substantially vertical orientation. A solid or liquid source of the compound is contained in the chamber or chambers. The ratio between the length of the chamber or combined length of chambers connected in series with respect to the direction of flow of the earner gas through the chamber or chambers and the average diameter equivalent of the cross section of the chamber or chambers with respect to the direction of flow of the carrier gas through the chamber or chambers is not less than about 6:1.