Flow visualizations and phase-averaged particle image velocimetry (PIV) measurements of a jet in crossflow configuration at very low Reynolds numbers (Rej ≃ 100) are performed in a water tunnel for jet-to-cross-stream velocity ratios R ranging from 1.5 to 4.5. The PIV vector fields and flow visualizations, carried out by injecting methylene blue dye and by the laser induced fluorescence (LIF) technique, are analysed to characterize the effect of R on the formation and evolution of large-scale vortices. It is shown that two distinct flow regimes are established depending on R, with R ≃ 3 being a transitional value. At low R, the longitudinal vorticity dynamics is dominated by the so-called wake-like structures which are shown to be strictly connected to the streamwise counter-rotating vortices (CRVP) which drive the destabilization of the jet flow. On the other hand, at large R, vortices with positive and negative vorticity are coupled together. The establishment of these different behaviours is interpreted physically as an effect of the jet Reynolds number which plays an essential role on the destabilization mechanisms which lead to the formation of the jet shear-layer structures. In any case, the onset of instability is driven by mechanisms which are different from those characteristic of free jets.