The formation of polyamide membranes via the interfacial polymerization (IP) of m-phenylenediamine (MPD) in water with trimesoyl chloride (TMC) in hexane or heptane is studied using two novel measurement techniques : light reflection and pendant-drop tensiometry. These techniques permit obtaining real-time data on effects of TMC and MPD concentration as well as reaction time on the IP membrane-formation process. The structure of the IP membranes is studied using scanning electron microscopy (SEM) and their performance is assessed via high pressure permeation measurements. These studies indicate that for very small TMC concentrations (< 0.01 wt%) the IP process is diffusion controlled in the organic layer. For higher TMC concentrations appropriate to commercial practice, the IP process is MPD-diffusion controlled in the IP film layer. Although under typical IP reaction conditions (MPD approximately 2 wt%, TMC approximately 0.1 wt%), the IP reaction is nearly instantaneous, the growth of the IP film thickness becomes self-limiting on a longer time scale (< 40 s) owing to the inability of the two reactants to interpenetrate after sufficient densification has occurred toward the organic side of the IP film. However, the IP film properties continue to change long after (> 300 s) the IP film growth becomes self-limiting presumably because of additional cross-linking. The light-reflection, pendant-drop tensiometry, SEM, and permeation measurements provide complementary supporting information on the IP membrane-formation process. In particular, these studies clearly indicate that the reactant concentrations and reaction time can be optimized to achieve improved rejection and permeation-flux properties. These studies also indicate that the light-reflection and pendant-drop-tensiometry measurement techniques provide very useful tools for studying the very rapid IP membrane-formation process.