High-vector-density planar velocity fields were obtained for a weakly compressible mixing layer using particle image velocimetry (PIV). The velocity ratio of the mixing layer was 0.53, the density ratio was 0.67, and the convective Mach number was 0.38. At the location where the PIV images were obtained, $\Re_x\,{=}\,3.7\,{\times}\,10^{6}$ and $\Re_{\delta_\omega}\,{=}\,1.8\,{\times}\,10^{5}$. The instantaneous planar velocity fields fall into three regimes characterized by the size and number of large-scale structures present. The large-scale rollers are either circular or elliptical, with the elliptical rollers having, in general, horizontal major axes. The transverse velocity fluctuations and Reynolds shear stress are suppressed for the weakly compressible mixing-layer as compared to the incompressible case. The spatial correlations of velocity fluctuations also occupy a smaller fraction of the mixing-layer thickness than for an incompressible mixing layer. The linear stochastic estimate of a roller structure is elliptical with the major axis oriented in the streamwise direction and with an eccentricity greater than for the incompressible case. The linear stochastic estimate of a braid suggests that the braids are vertically oriented, as opposed to the oblique orientation seen in incompressible mixing layers. In addition, the braids in the weakly compressible case have a vertically oriented stagnation line, as opposed to the braids in the incompressible mixing layer where stagnation occurs at a point.