The goal of this paper is to describe the development of the sound database for the allophone-based model for English concatenative speech synthesis. The procedure of the sound unit inventory construction is described and its main results are presented. At present moment the optimized sound units inventory of the allophonic database for English concatenative speech synthesis contains 1200 elements (1000 vowel allophones and 200 consonant allophones). The smoothness of junctions between the allophones shows high quality of the segmentation made. The decrease in the number of the database components in the result of optimization does not affect the quality of the resulting synthesized speech. At the level of segments it can be evaluated as fairly high in terms of both naturalness and intelligibility.