Heat-transfer coefficients for turbulent airflow in a circular tube situated downstream of a mixing tee have been determined experimentally. In the experiments, perpendicularly oriented air streams were respectively ducted to a tee via its center port and a side port. The merged flow exited the tee from the other side port and passed into the heated test section tube. From the experimental data, results were obtained for the circumferential average heat-transfer coefficient at a succession of axial stations and for the circumferential variation of the transfer coefficient at each such station. It was found that the mixing and turning of the flow gives rise to a substantial augmentation of the heat-transfer coefficients compared with those in a conventional thermal entrance region; the thermal entrance length is also substantially elongated. At a given Reynolds number, the axial distributions of the circumferential average coefficient are bounded between the distributions for the two limiting cases of no mixing (i.e. entire flow entering either via the side port or via the center port). When the axial distributions are normalized by the corresponding fully developed values, they are nearly insensitive to the Reynolds number. The circumferential variations are largest just downstream of the tee. In most cases, the variations decay to the ± 3% level at 12 diameters downstream of the tee, but in some instances residual variations persist at 40-60 diameters.