The active control problem of gaseous systems is addressed in this article. The objective is to regulate gas velocity, at particular locations within the system, so that appropriate mass flow rate is achieved. The dynamics describing the system, using modal expansion, reduce to a finite set of ordinary differential equations. Due to variations of the gas properties with operating conditions, there exists parametric uncertainty in the obtained reduced-order model. To overcome the problem of estimating the model parameters or the extent of the parametric uncertainty, a variable structure controller is proposed. This controller utilizes a fuzzy logic rule base for on-line adjustment of the switching gain. The fuzzy rules create an adaptive law and tune this gain to the smallest value that verifies the sliding condition. Experimental results demonstrate the performance of the suggested mass flow rate control scheme, tested on a prototype air handling unit.