The coalescence phenomena of a pair of air bubbles was studied in anionic/cationic (SDBS/DTMAC) mixed surfactant solution by an optical probe method in comparison with the single surfactant solution. For dimensionless concentration ($Qrk^2$/σ) greater than 400, Dynamic surface tension model could be applicable to describe bubble coalescence time in single and binary surfactant solution. The synergistic effect of the surface activity in SDBS/DTMAC solution was verified by the increase of coalescence time and the decrease of transition concentration. Film-thinning simulation model was developed for predicting the effect of parameters influencing bubble coalescence such as solution concentration, bubble size and temperature, and the results have similar tendencies compared with the experimental results. The prediction of precipitation phase boundary of anionic/cationic (NaDS/DPCl) binary surfactant solution could be done from the coalescence time data of that solution.
