There has recently been considerable interest in the development of devices for the preparation of monodisperse microbubble suspensions for use as ultrasound contrast agents and drug delivery vehicles. These applications require not only a high degree of bubble uniformity but also a maximum bubble size of 8 mu m, and this provides a strong motivation for developing an improved understanding of the process of bubble formation in a given device. The aim of this work was to investigate bubble formation in a T-junction device and determine the influence of the different processing parameters upon bubble size, in particular, liquid viscosity. Images of air bubble formation in a specially designed T-junction were recorded using a high-speed camera for different ratios of liquid to gas flow rate (Q(l)/Q(g)) and different liquid viscosities (mu(1)). It was found that theoretical predictions of the flow profile in the focal region based on analysis of axisymmetric Stokes flow were accurate to within 6% when compared with the experimental data. indicating that this provided a suitable means of describing the bubble formation process. Both the theoretical and experimental results showed that Q(l)/Q(g) and mu(1) had a significant influence upon bubble formation and eventual size, with higher flow rates and higher viscosities producing smaller bubbles. There were, however, found to be limiting values of Q(l)/Q(g) and mu(1) beyond which no further reduction in bubble size was achieved.