A pseudoternary system exhibiting a single-phase domain extending from water-in-decane to sponge phase to decane-in-water microemulsions as temperature and decane weight fraction (alpha; relative to brine+decane) are varied has been studied by Chen, Chang, and Strey [J. Chem. Phys. 93, 1907 (1990)]. Conductivity and self-diffusion measurements at 45 degrees C show that the reverse micelle to sponge phase transition proceeds in a sequence of two continuous transitions defined quantitatively by an order parameter. This is the first study to quantify the onset of sponge phase formation within a single-phase domain, as a function of composition, and to quantify the amount of sponge phase.