The term "Viscous fingering" is usually used to refer to the instabilities that occur at the interface when a more viscous fluid is displaced by a less viscous fluid.However, we succeed an experimental study of viscous fingering fully triggered by a chemical reaction involving a viscosity increase and decrease when a less viscous fluid is displaced by a more viscous fluid in a radial Hele-Shaw cell. This is done by making use of a polymer (polyacrylic acid , sodium polyacrylate) solution's dependence of viscosity on pH. We show that a difference in fingering patterns is observed, depending on whether the viscosity is increased or decreased by the reactions.