This research utilized the planar laser-induced fluorescence technique and high-speed photography to determine the mixing characteristics of equal-momentum doublet impinging water jet sprays in fully developed conditions. The injector is composed of two 0.3 mm diameter orifices at +30 deg/-30 deg impinging angle. The penetration percentage (P.P.) of droplets passing the center plane of the sprays front either jet is identified to characterize the mixing phenomena. In this experimental setup, the best mixing occurs at the P.P. of 50%, however, the experimental observed P.P. were all >50%, and that indicates the effects of convection and diffusion on liquid mixing were fully relaxed. The analyses showed that the state of the jets at the impinging point has crucial effects on the PP. From high-speed photography, the growth and the propagation of the waves on liquid jets are the controlling factors. The P.P. variation with momentum flux in fully developed conditions are induced by jet momentum flux fluctuation at impingement caused by the onset of axisymmetric waves on free jets, and by the skew impingement resulted from the growth of helical waves on jets at impinging point.