The viscosity control of PVAc latex in the presence of PVA as a protective colloid based on an initiator has been investigated. The polymerization was carried out not by the former techniques such as speed of stirrer, PVA amount, kind of PVA, and charged VAc amount, etc., but by the new and simple means such as the amount of an initiator and its addition method. In polymerization initiated by HPO, the viscosity of PVAc latexes was easily controlled to obtain the thixotropy, Newtonian, or dilatancy flow, and also an increase or a decrease, by means of the amount of HPO and its addition method. It may be concluded that the viscosity of PVAc latexes containing PVA is determined mainly by the initiator amount at an early stage of polymerization. In other words, the particle number depends on the initiator amount at an early stage of polymerization, because particles produced are effectively protected by PVA. The confirmation of the graft copolymer between PVA and PVAc undertaken by the extraction method stands in need of reexamination, as the residual polymers increase more with smaller initiator amount. Through a series of studies, the PVA-PVAc composite porous membrane was prepared by extraction of PVAc with acetone from the film of the PVAc latex polymerized in the presence of PVA by HPO. The membrane consists of spherical cells which were made up of PVA and grafted PVAc or insoluble PVAc-like microgels. Unexpectedly, the PVA-PVAc composite porous membrane is permeated by n-hexane but is not appreciably permeated by benzene. Based on these properties, a new use beyond adhesives, paints, and textile finishes is now being found for PVAc latex containing PVA.