Circular-to-rectangular transition ducts are used on aircraft with rectangular exhaust nozzles to connect the engine and nozzle. Often, the incoming flow of these transition ducts includes a swirling velocity component remaining from the gas turbine engine. Previous transition duct studies have either not included inlet swirl or when inlet swirl was considered, only overall performance parameters were evaluated. This dissertation reports the study of circular-to-rectangular transition duct flows with and without inlet swirl. This study was completed in order to understand the effect of inlet swirl on the transition duct flow field and to provide detailed duct flow data for comparison with numerical code predictions;A method was devised to create a swirling, solid body rotational flow with minimal associated disturbances. Details of the swirl generator design and construction are discussed. Coefficients based on velocities and total and static pressures measured in cross stream planes at four axial locations within the transition duct along with surface static pressures and surface oil film visualization are presented for both nonswirling and swirling incoming flows. For nonswirling flow, measurements were recorded for three inlet centerline Mach numbers; 0.20,0.35, and 0.50. The centerline Mach number for the swirling flow measurements was 0.35. The maximum ideal swirl angle was 15.6;A method was developed to acquire trace gas measurements within the transition duct at high flow velocities. Trace gas results are presented for the case of nonswirling incoming flow with an inlet centerline Mach number of 0.50. Statistical methods are used to help interpret the trace gas results;Inlet swirl significantly changes the transition duct flow field. For nonswirling flow, the distribution of static pressure is attributed to the response of the flow field outside the boundary layer to the changing duct geometry. With swirl, the static pressure distribution is additionally affected by the streamline curvature of the swirl. For nonswirling incoming flow the static pressure distribution produced skew-induced secondary flows within the boundary layer that evolved into two pair of counter-rotating side wall vortices. These vortices were absent in the swirling incoming flow case. The results show the effects of inlet swirl should be included in the design and numerical analysis of circular-to-rectangular transition ducts for aircraft exhaust systems.