One of the most widespread canonical devices for fluid mixing is the T-shaped mixer, in which two opposing miscible liquid streams meet at a junction and then mix along a main channel. Laminar steady and time-periodic flows in T-shaped mixers have been thoroughly studied, but turbulent flows have received much less scrutiny despite their prevalence in applications. We here present optical measurements of turbulent mixing at small scales in a novel experimental setup with a hydraulic diameter of four centimetres. Water is used as a working fluid and the Reynolds number,Re, based on the bulk velocity and hydraulic diameter ranges from the laminar (Re=100) to the fully turbulent case (Re=5000). The data comprises two-dimensional particle image velocimetry (PIV) of the velocity field and planar laser-induced fluorescence measurements (PLIF) of the passive scalar (Rhodamine 6G). First, we successfully replicate characteristic flow regimes observed in micro-scale T-shaped mixers at low Reynolds numbers. We then focus on the turbulent regime and characterize the turbulent kinetic energy and dissipation along the mixing channel. Further, we determine the scalar concentration variance and its corresponding probability density function and spectra. The latter exhibits an incipient Batchelor scaling. We estimate the mechanical-to-scalar timescale ratio and examine the link between the turbulent velocity and scalar fields. The measurement data are compared with model predictions and correlations used in engineering practice, and with data from our own direct numerical simulations at Re=1500 performed with a spectral-element code.