The efficiency and mechanisms of cooling a constant heat flux surface by impinging synthetic jets were investigated experimentally and compared to cooling with continuous jets. Effects of jet formation frequency and Reynolds number at different nozzle-to-surface distances (H∕d) were investigated. High formation frequency (f=1200Hz) synthetic jets were found to remove heat better than low frequency (f=420Hz) jets for small H∕d, while low frequency jets are more effective at larger H∕d. Moreover, synthetic jets are about three times more effective in cooling than continuous jets at the same Reynolds number. Using particle image velocimetry, it was shown that the higher formation frequency jets are associated with breakdown and merging of vortices before they impinge on the surface. For the lower frequency jets, the wavelength between coherent structures is larger such that vortex rings impinge on the surface separately.