We performed an experimental and numerical investigation of a convective buoyancy-driven instability that arises during the injection of a denser miscible fluid into a less dense one in a rectilinear geometry. We visualized the instability using a shadowgraph technique, and we obtained quantitative information using micro-Particle Image Velocimetry. Numerical simulations provided further insights into the three-dimensional (3D) velocity field. We have shown that the instability only occurs above a certain Péclet number, Pe, depending on the Rayleigh, Ra, and Schmidt, Sc, numbers. We suggest scalings of the critical time, TC, and dimensionless wavelength, λ / h, of the instability, both of which increase with increasing Pe and Ra. Finally, we investigated the interactions of the instability vortices with each other and the geometry boundaries.