The thermal-hydraulic design characteristics of a boiling water reactor are strongly dependent on the physics of heat-removing mechanisms from nuclear rods. Performance is often limited by a lack of understanding of the heat and momentum transfer in thin liquid films close to dryout. This is particularly important with regard to power uprates of reactors. Flow control by functional spacers equipped with vanes adds complexity to the flow behavior. Instead of improving the heat removal, they can cause a local reduction of the film thickness under unfavorable conditions. This work presents a novel experimental technique to measure the velocity of the liquid phase and to study the turbulent mixing of a passive scalar in the film. The studies were performed in vertical annular flow in a double-subchannel geometry. The liquid film was labeled by either continuous or pulsed tracer injection. An electrically conductive tracer (salt solution) is injected into the film consisting of deionized water. The salt is traced using a liquid film sensor, a high-frequency nonintrusive conductivity-based technique. The experiments are conducted with and without a swirl-type spacer to quantify the effect of the spacers on the film flow for different gas and liquid flow rates. An ensemble averaging of the data obtained from individual injection pulses provides time-averaged liquid velocities in the film. The turbulent dispersion was characterized by analyzing the spread of the tracer in axial and lateral directions. Spacer vanes are found to induce a characteristic transverse movement of the liquid in the film. The effect of waves on mixing is analyzed and is found to enhance it.