The aim of this thesis is to find out the effects of speech and speech intelligibility on computer-based task performance in open-plan offices. The research was conducted in a real open-plan office environment to include the open-office experience of subjects to the analysis. The experiment consists of three main phases. In the first phase, real-size measurement of selected open-office area within STM Bilkent was analyzed to understand effects of divider panels on acoustical situation of the room, and to check the reliability of the computer simulation. In the second phase, acoustical simulation of the site was done, to derive distribution graphs for speech related room acoustics parameters. In the last phase, occupants computer-based task performances were tested under three different sound environments, which are continuous noise, speech and masked speech. According to statistical analysis of performance test, and acoustical properties of the case STM, suggestions for renovation were discussed and tested in computer simulation.