Hot‐wire anemometry and digital techniques are used to develop time series for the streamwise and transverse velocity components and for the mass fraction of helium in the low‐speed turbulent boundary layer of air involving the slot injection of helium. Profiles are developed at several streamwise stations. In the present contribution the data are presented principally in terms of conventional, unconditioned averages. It is found that the mean velocity and concentration profiles develop in the downstream direction in the expected fashion but that the intensities of the velocity, concentration, and density fluctuations increase sharply downstream of the slot and peak in the region where the mixing region from the splitter plate impinges on the wall. Close to the wall where the helium concentration is relatively high counterfluxes of helium are found. The data at one streamwise station are reanalyzed in terms of Favre‐averaging so that the two means of averaging can be compared in a flow involving significant density fluctuations.