In open plan offices the lack of speech privacy between the workstations is one of the major acoustic problems. Improving the speech privacy in an open plan design is therefore the main concern for a successful open plan environment. The project described in this paper aimed to find an objective parameter that correlates well with the perceived degree of speech privacy and to derive a clear method for evaluating the acoustic conditions in open plan offices. Acoustic measurements were carried out in an open plan office, followed by data analysis at the Acoustic Department, DTU. A computer model of the actual office was developed using the ODEON room acoustic software, and this allowed a systematic investigation of the possible influence of various acoustic conditions on the speech privacy. Four different versions of acoustic treatment of the office were used and three different distances from the speech source. Listening tests were performed with the sound files obtained from the simulations, in all cases including a masking noise from a ventilation system. It was found that the Speech Transmission Index (STI) is a good descriptor of perceived speech privacy and that values below 0.30 are required in order to obtain a “moderately good” speech privacy condition.