<p>In this study, mass-transfer coefficients and flow fields were experimentally investigated at a T-shaped pipe junction to clarify the thinning characteristics in a T-junction. To understand the relationship between the flow pattern and mass-transfer characteristics, experiments were conducted at different flow rates between the main and branch pipes. The calcium-sulfate dissolution method was used to evaluate the mass-transfer characteristics, and particle image velocimetry was used to measure the flow field. The experimental results show that when an inflow occurs from both the main and branch pipes, the mass-transfer-coefficient distribution in the T-junction exhibits two peaks on the wall of the main pipe on the side where the branch pipe is connected. The location of the peaks depends on the flow separation and reattachment, and a similar distribution shape is obtained regardless of the flow-rate ratio. Measurements of the velocity field reveal that secondary flows generated by flow separation at the confluence cause a local increase in the mass transfer coefficient. When only an inflow occurs from the branch pipe, the distribution of the mass-transfer coefficient changes significantly. The mass-transfer coefficient on the wall of the main pipe is higher than that on the branch pipe. This is because the flow from the branch pipe collides with the wall of the main pipe. Under this condition, unlike the case when flow enters from both pipes, the biased flow originating at the confluence persists well downstream. As a result of this biased flow effect, a locally high mass transfer coefficient is maintained in the systems where flow enters only from the branch pipe. The correlation between the flow field and mass transfer elucidates the respective contributions of mean velocity and turbulent kinetic energy to enhancing mass transfer in a T-pipe.</p>