A series of experiments were conducted in water to study mixing in the field of a single, two-dimensional vortex. The experimental configuration is that of a laminar line vortex initiated along a diffusion layer between two streams of different scalar concentrations. Measurements of passive scalars in inert and chemically reactive environments were made using a planar laser-induced fluorescence technique. A fast acid/base isothermal reaction was utilized to highlight the molecular mixing. The experimental results show that the mixing enhancement in the presence of a vortex is linearly dependent on the vortex strength and the time elapsed since vortex initiation. In particular, the mixedness, defined as the spatially integrated second moment of concentration field in the vortex, and the spatially averaged scalar dissipation are found to follow this dependence. This variation is mainly attributed to the contact area generation along the diffusion layers between the two streams as a result of inviscid deformations in the vortical flow field. The results presented pertain to mixing in liquids and in the limit of high Schmidt numbers.