To provide an emulsion that has increased stability without deterioration in emulsifiability for a long period of time, even under acidic or elevated- temperature conditions and is useful as a base for pressure-sensitive adhesive, binder, agrochemical or waterproof agent for fiber or woven fabric.

This polyisobutylene emulsion comprises (A) 100 pts.wt. of polyisobutylene with a viscosity-average molecular weight of 10,000-70,000, (B) 1-30 pts.wt. of polyoxyethylene-oxypropylene block copolymer wit a number - average molecular weight of 1,500-80,000, (C) 0.1-5 pts.wt. of polyoxyethylene alkyl ether sulfate ester salt or, in addition, (D) 0-2 pts.wt. of stabilizers where the solid content is 1-75wt.% and the rest is water in the emulsion.