In this work, the effects of the pre-emulsion feed rate on the instantaneous and overall conversion, average particle diameter, particle size distribution, polymer particle concentration, polymerization rate, average number of radicals per particle, viscosity and coagulum of a 65 wt% total solid contents of butyl acrylate/vinyl acetate copolymer latexes during relevant emulsion polymerization over pre-emulsion feeding times of 1, 2, 3, 4 and 6 h were investigated. The temperature of reaction was maintained at 85 degrees C during the feed input and at 90 degrees C for I h after the end of the feed. The effect was quite significant for feeding times of I to 3 h and it was of minor significance for feeding times varying from 3 to 6 h for the conversion, average particle diameter and polymer particle concentration. The polymerization rate was influenced by the feeding time, while the particle size distribution was not affected by the feeding time. The viscosity and average number of radicals per particle exhibited a clear tendency of inversion at the critical feeding time of 3 h. The coagulum formation decreased with feeding time up to 4 h and increased drastically for 6 h.