Abstract Chemical reactions in time-periodic 2D chaotic flows are examined by solving numerically the convection-diffusion-reaction equation. Three flow conditions are considered: a predominantly regular system, a predominantly chaotic system with a few regular islands, and a globally chaotic system devoid of noticeable islands. Chaotic mixing has a strong impact on systems undergoing a single bimolecular reaction A + B → C. Maximum concentration of C occurs in well-mixed chaotic regions; in comparison, little reaction takes place inside poorly mixed non-chaotic islands. Chaotic mixing also has a significant impact on competitive-consecutive reactions A + B → P, B + P → W. The relative amounts of P and W generated by the reactions are strongly affected by the presence of islands. While the desired product P predominates in chaotic regions, most of the waste W is generated inside islands.