This thesis dealt with the traditionally problematic standard Croatian jat. Firstly, an overview of the current thoughts and discussions about the standard jat was presented, divided into the problem of writing, the problem of pronunciation, the problem of morphonological behavior, the problem of phonemic status, and the problem of the communicative style status of the speech sound jat. After that the research portion of the thesis was described. The goals of the research were to compare the acoustic characteristics of the standard Croatian jat and the sequence [je], to examine the iotation of the sounds [n] and [l] by the standard jat, to compare the acoustic characteristics of the standard jat with those of the Kajkavian diphthong [i͡e], and to examine the auditory perception of the standard jat in comparison with the sequence [je]. The hypotheses were that the jat sound would not differ acoustically from the sequence [je], that the iotation of the sounds [n] and [l] by the long jat will occur in the small number of cases, that the standard jat will have a wider initial distance between F1 and F2 than the Kajkavian diphthong [i͡e], and that Croatian speakers will not perceive the difference between the jat sound and the sequence [je]. The inconsistent and complex acoustic-perceptual image of the standard jat formed by the results supports the claim that jat is not a phoneme. The acoustic analysis and comparison with the Kajkavian diphthong indicates that the materialization of jat is not a diphthong.