Flow field distributions and drug dispersion inside the mouthpiece of a Spiros((R)) inhaler, a dry powder inhaler (DPI) delivering therapeutic powders to airways, were investigated by means of the particle image velocimetry (PIV) technique. The aim of the study was to obtain flow patterns inside the mouthpiece during a typical inhalation cycle, and to gain insight on the powder dispersion mechanism inside the aerosolization chamber. Air stream velocities inside the mouthpiece were determined from correlated image-pairs of smoke particles, introduced around the modeled inhaler and drawn into the mouthpiece. On the other hand, to investigate the dispersion mechanism, lactose particles (the excipient) were replaced by fluorescent particles that bad a similar mean aerodynamic diameter, so that differentiation from the Mie scattering signal of the drug particles could be made by using high-pass and band-pass color filters. The two emissions were simultaneous recorded using a pair of digital cameras, and the dispersion of each was recorded at various normal planes within the mouthpiece. Experimental results showed that rotational turbulent flow prevailed in the downstream sections of the mouthpiece. Impaction seemed to be the dominant dispersion mechanism in the modeled inhaler and the respirable fraction of drug particles was released from the dispersion chamber within the first 2 s of operation.