Visual observations of coalescence of n-hexadecane oil drops (70-140 mu m in diameter) suspended inside an aqueous phase of varying pH are reported. The oil drops are produced in situ and confined inside the aqueous-phase-filled fine capillary (100-160 mu m i.d.). Rapid, spontaneous coalescence was observed at low pH (less than or equal to 2.5) as soon as two drops were very carefully brought into apparent contact. At very high pH (greater than or equal to 13.0), coalescence was observed some time after the drops made contact. No coalescence was observed at intermediate pH 5.2, 7.1, and 10.9. The "coalescence times" at pH 2.2 and 2.4 were less than 0.033 s. At pH 3.2, 12, and 13.1, the coalescence times were 8, 20, and 1.8 s, respectively. The coalescence time in the acidic solution of a certain pH was distinctly lower than that in the basic solution of equivalent pOH. The coalescence tendency increased with an increase in the acidity or basicity of the aqueous phase. Also, in strongly basic or acidic media, freshly formed drops were more susceptible to coalescence. Visual observations of collision interactions are qualitatively described. When two drops were made to collide, coalescence was rapid at all pH at which coalescence was previously observed for the stationary drops. Almost no coalescence was observed at the intermediate pH’s 5.2, 7.1, and 10.9, even after repeated collisions between the drops. The relative velocity of the collision interactions ranged from 100 to 2100 mu m/s. The observed phenomena are explained on the basis of the London-van der Waals and electrical double layer forces, which are important either in stabilizing the thin liquid film formed between the approaching drops or in drawing the two interfaces close enough to thin the liquid film to the "critical rupture thickness range." In this range, it is believed that thin liquid him properties control the rupture process.