Flow reattachment and separation over a thick airfoil is dynamically controlled using synthetic jet actuators. The response of the flow to the control input is measured in the cross stream plane of the airfoil wake using phase-locked two-component hot-wire anemometry. At alpha=15^o (Re_c=310,000), the uncontrolled flow is separated and the dimensionless (natural) shedding frequency is F^+=0.7. In the present experiments, control is effected using jet formation frequencies that are either well above (F^+=10) or of the same order as (F^+=0.9) the natural shedding frequency. For both frequencies, the collapse of the separated flow region is associated with a strong momentary reduction in lift that is marked by the formation and shedding of a vortex having vorticity of the same sense as on the top (suction) surface. The subsequent recovery is accompanied by the shedding of a stronger, opposite sign vortex ultimately leading to a substantial increase in the mean lift (and a corresponding reduction in pressure drag) which is approximately the same for both actuation frequencies. However, while at F^+=10 the shedding of organized vortical structures subsides following the initial transient, actuation at F^+=0.9 leads to a time-periodic shedding of a train of vortices (at the actuation frequency) that correspond to (peak to peak) lift coefficient fluctuations of up to 60% of the mean.