Surface properties and aggregation behaviour of mixed surfactant systems with varying molar ratios of hexadecyltrimethylammonium bromide (HTAB) and palm-based caprylic acid (OA) were studied in the present work. The critical micelle concentration (CMC), surface excess concentration (Гmax), minimum area per molecule (Amin), surface pressure at CMC (πCMC), zeta potential and particle size were determined using instrumentations (e.g., surface tensiometer and particle sizer). CMC of HTAB-OA mixed surfactant systems was about 30% (0.532 mM) to 50% (0.359 mM) lower than HTAB single surfactant system (0.787 mM), indicating better surfactant adsorption efficiency. Results also showed that increasing the molar ratio of OA favoured the formation of micelles at low concentration; thus, lowering both CMC and surface tension. However, Гmax of mixed surfactant systems lowered with OA molar ratio, indicating the mixed surfactant systems tend to form stable ion pairs in bulk solution rather than orientating at the air-water interface. CMC values obtained via surface tension measurement were double confirmed by ultraviolet-visible (UV-VIS) absorbance measurement using dye solubilisation. The particle size of mixed surfactant aggregates (250 ± 50 nm) was 50 times larger than the one found in HTAB surfactant (5.6 ± 0.3 nm). Equimolar mixed surfactant system displayed highest colloids stability and aggregates populations. The results suggested the existence of strong synergism when two oppositely charged surfactants are mixed.